Roast Lamb with Kūmara.

Name: Roast Lamb with Kūmara (Kiwi Sunday Roast)
Prep time: 25 min
Cook time: 1 h 30 min
Total time: 2 h 15 min
Yield: 6 servings
Cuisine: New Zealand
Ingredients: 2 kg bone-in leg of lamb; 6 cloves garlic, cut into slivers; 4 rosemary sprigs, leaves picked into short lengths; 2 tbsp olive oil; 2 tsp flaky salt; 1 tsp coarsely ground black pepper; 900 g kūmara, in large chunks; 700 g floury potatoes, halved; 2 red onions, cut in wedges; 1 cup mint leaves, finely chopped; 80 ml malt or white wine vinegar; 2 tsp caster sugar; 3 tbsp boiling water

Kiwi Sunday Roast

· say it: roast lam with KOO-ma-ra

This is the Sunday roast New Zealand built around its most famous export: a bone-in leg of lamb, slit and packed with garlic slivers and rosemary, set over a tray of kūmara and potatoes so the fat drips down and glazes them while the meat roasts. The lamb comes out rosy in the middle with a salt-crusted outside; the kūmara goes soft and slightly caramelised at the edges where the sugars catch. Mint sauce — thin, sharp, vinegary, not the sweet jelly — cuts the fat and is non-negotiable at most tables. It takes one tray, about an hour and a half of oven time, and almost no attention while it cooks.

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Kiwi Sunday Roast

Gate: New ZealandCanterbury and Hawke's Bay lamb country
Prep: 25 min
Cook: 1 hr 30 min
Total time: 2 hr 15 min incl. 20 min rest
Serves: 6
Status: Medium
Nutrition / serving · estimated: Calories 680 kcal; Protein 46 g; Carbs 45 g; Fat 34 g; Fiber 6 g; Sugar 12 g

Why this recipe works.

One tray does everything — the lamb fat bastes the kūmara and potatoes, so there is no separate roasting fat to manage.
Gives temperatures for both pink and well-done lamb instead of assuming everyone wants the same thing.
Real mint sauce, made in two minutes from vinegar and sugar, not the sweet jelly sold in jars.
A long, uncovered rest that lets the juices settle so the carving board stays dry and the meat stays juicy.

Tips from our kitchen.

Take the lamb out of the fridge a full hour before roasting. A cold leg cooks unevenly and the outside overshoots while the centre lags.
Cut kūmara larger than the potatoes. It is softer and cooks faster, and matched sizes leave you with mush and hard potatoes.
Use a meat thermometer in the thickest part of the leg, away from the bone. Timing charts are guesses; temperature is not.
Rest the lamb at least 20 minutes on a warm board, loosely tented. The internal temperature will climb another 3–5°C as it rests.
Make the mint sauce while the lamb is in the oven so the mint has time to soften in the vinegar and lose its raw edge.
If the tray looks dry halfway through, add a splash of water rather than more oil — it stops the juices catching and becoming bitter.

Substitutions.

Kūmara → Orange sweet potato, or a mix of sweet potato and parsnip: Kūmara is starchier and less sweet, so parsnip in the tray brings the balance closer.
Bone-in leg of lamb → Boned and rolled leg, or lamb shoulder: A rolled leg cooks faster and carves easily. Shoulder needs a slower, longer roast and will not be pink — cook it to falling-apart instead.
Fresh mint for the sauce → Two tablespoons of good bought mint sauce, thinned with extra vinegar: Bought sauce is usually too sweet and too thick straight from the jar. Thinning it fixes most of that.

Variations.

Anchovy and rosemary lamb: Push a sliver of anchovy in with each garlic clove. It melts entirely and leaves no fishiness, only a deeper savoury edge to the meat.
Slow-roast shoulder: Swap the leg for a shoulder and roast at 150°C for four hours, covered. It pulls apart with a spoon instead of carving, and suits a crowd.
Hāngī-style lamb: Add smoked salt to the rub and a tray of water below the rack for the first hour, borrowing the damp, smoky character of the earth oven.

Storage & reheating.

Carved lamb keeps 3 days refrigerated. Store it in slices with a spoonful of pan juices poured over so it does not dry out, and reheat only briefly at 150°C — hot lamb reheated twice goes grey and tough. Cold roast lamb sandwiches with mint sauce are arguably the point of cooking a whole leg. Roast kūmara does not reheat well and is best used cold in a salad or fried in a pan with butter until crisp.

Serve with

Mint sauce, pan gravy made in the roasting tray, and buttered peas or steamed silverbeet.

The story behind Roast Lamb with Kūmara.

For most of the twentieth century, New Zealand had far more sheep than people — the ratio peaked at over twenty to one in the early 1980s — and lamb was both the country's economic backbone and its default Sunday dinner. Refrigerated shipping, first sailed successfully from Port Chalmers to London in 1882, is the fact that made it possible: it turned a distant pastoral colony into Britain's meat supplier and set the pattern of raising grass-fed lamb for export that still shapes the industry. The roast on the family table was, in a real sense, the same animal being sent to the other side of the world.

The Sunday roast itself is inherited from Britain, and the early colonial version was fairly literal: leg of lamb, roast potatoes, boiled vegetables, mint sauce, gravy. What made it local was kūmara. The sweet potato Māori brought south by canoe and grew in specially built warm pits was already a staple long before Europeans arrived, and it slid into the roasting tray alongside the potatoes without anyone announcing it as fusion. A New Zealand roast tray with no kūmara in it looks slightly wrong to most people who grew up there.

Lamb is no longer the cheap default it once was — flock numbers have fallen by well over half since the 1980s, and a leg is now a considered purchase rather than a weekly one. That has changed how it is cooked more than what is cooked. Where the older tradition roasted lamb well past pink, in the British manner, most New Zealand cooks now pull it out at medium-rare and rest it hard, which suits grass-fed meat far better. This recipe follows the modern habit, and gives the timings for the older one, because plenty of families still want their lamb cooked through and there is no point pretending otherwise.

Questions, answered.

What internal temperature should roast lamb be?

Pull the lamb from the oven at 55°C for medium-rare, 60°C for medium and 68°C for well done, measured in the thickest part of the leg away from the bone. It will rise another 3 to 5°C while resting, so taking it out at the target temperature means overshooting it.

What is kūmara and can I use ordinary sweet potato?

Kūmara is the New Zealand sweet potato, brought to Aotearoa by Māori voyagers and grown there for centuries. It is drier, starchier and less sugary than the orange sweet potato common elsewhere. Ordinary sweet potato works in the tray but caramelises faster and tastes noticeably sweeter, so cut it a little larger.

Why does my roast lamb come out tough?

Two usual causes: it went into the oven straight from the fridge, or it was carved without resting. A cold leg cooks unevenly, and cutting into hot meat lets the juices run onto the board instead of settling back into the muscle. Rest it a full 20 minutes.

Can I cook the vegetables in the same tray as the lamb?

Yes, and it is the whole idea here — the lamb fat drips onto the kūmara and potatoes and does the basting for you. Sit the lamb on a rack or directly on the vegetables, and stir the tray once so the pieces underneath get their turn in the fat.

Should mint sauce be sweet?

Only slightly. Traditional mint sauce is chopped mint steeped in hot vinegar with just enough sugar to take the edge off — thin, sharp and pourable. The thick green jelly sold in jars is a different, much sweeter product and works against the fat of the lamb rather than cutting it.

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New Zealand2 hr 30 min