Ingredients

Equipment Needed

• •Heavy ovenproof pan or roasting tin
• •Very sharp knife for scoring
• •Meat thermometer
• •Warm serving platter or carving board

Instructions

1. 1

   Temper and score the lamb

   Take the lamb out of the fridge a full hour before you cook it. Cold meat in a hot oven seizes, the outside overcooks before the centre warms, and you end up with grey edges around a cold middle. Room temperature is where even cooking begins. While the lamb tempers, lay it fat-side up on a board and score the fat cap in a crosshatch pattern with a very sharp knife. You're cutting through the fat only, not into the meat beneath. The lines should be about 1cm apart. These grooves do two things: they let the fat render faster in the oven's heat, and they give the salt and mustard a way into the meat. Without them, the crust sits on the surface and slides off the moment you carve.

   If you feel the resistance change under your knife, you've gone too deep. The fat feels soft and waxy. The meat pushes back. Stop at the push.
2. 2

   Season generously

   Rub coarse sea salt into the scored fat cap, pressing it down into every groove with your fingers. Use more than feels polite. Coarse salt here is doing two jobs: it pulls moisture from the surface of the fat so it crisps instead of steaming, and it seasons the meat from the outside in through every cut line you've made. Season the underside and the edges with salt and freshly cracked black pepper. The coarseness of the salt matters. Fine salt dissolves too fast and doesn't give the crust its texture.

3. 3

   Sear the fat cap

   Heat the rapeseed oil in a heavy ovenproof pan or roasting tin over medium-high heat. When the oil shimmers, lay the lamb in fat-side down. Don't touch it. Let it sear for four to five minutes until the fat is deep golden and the crosshatch lines have tightened and crisped at the edges. You'll hear it crackling. That sound is the fat rendering and the surface browning, both of which are flavour. Lift the lamb out and set it aside on a board, fat-side up. Pour off most of the rendered fat but don't clean the pan. Those brown bits on the bottom are your sauce later.

   Rapeseed oil, not butter, for this sear. Butter burns at this temperature and the fat cap needs time to render. The lamb provides its own richness. The oil is just there to start the process.
4. 4

   Build the mustard crust

   In a small bowl, mix both mustards with the minced garlic, thyme leaves, breadcrumbs, and the melted butter. Stir until you have a rough, slightly sticky paste. The Dijon gives heat and sharpness. The coarse-grain gives texture and a gentler,seedy warmth. Together they form a crust that browns in the oven without burning because the breadcrumbs absorb the moisture and the butter helps them crisp. This is the skorpe, and it's what makes the dish.

5. 5

   Apply the crust and roast

   Heat the oven to 220C. Press the mustard mixture firmly onto the seared fat cap, packing it into the scored lines. It should coat the top and edges in a thick, even layer. Don't be delicate. Press it on like you mean it, because a loose crust falls off during carving. Place the lamb crust-side up in the same pan, scatter the thyme sprigs around it, and put it straight into the hot oven. Roast for 25 minutes for pink meat, 30 for medium, 35 if you want it cooked through. I'll tell you now: this lamb is best at pink. The mustard crust wants something tender and rosy underneath it, not something grey.

   A meat thermometer takes the guessing out. Pull the lamb at 55C for pink, 60C for medium. It will climb another five degrees while it rests.
6. 6

   Rest the lamb

   This is the step that people skip, and it's the step that matters most. Lift the lamb onto a warm plate or board and cover it loosely with foil. Let it rest for fifteen minutes. Not five. Fifteen. Resting lets the muscle fibres relax and reabsorb their juices. If you carve too soon, those juices pour out onto the board and the meat is dry. After fifteen minutes, the lamb will be warm, pink, and juicy all the way through. The crust will have set and firmed. You'll know when it's right.

7. 7

   Make the pan sauce

   While the lamb rests, set the roasting pan over medium heat on the hob. Pour in the white wine and let it bubble, scraping the brown bits from the bottom with a wooden spoon. These are the fond, the concentrated flavour of the sear and the roast, and they dissolve into the wine as it reduces. When the wine has reduced by half, add the stock and let the whole thing simmer for five or six minutes until it coats the back of a spoon. Take it off the heat and stir in the cold butter. The butter emulsifies the sauce, giving it body and a gentle gloss. Strain through a sieve into a warm jug. Season with salt and pepper. This is a light sauce, not a heavy gravy. It should taste of lamb and mustard and thyme.

8. 8

   Carve and serve

   Carve the lamb into thick slices, about 1cm, cutting across the grain so each slice is tender. The crust should hold as a golden cap on each piece, the meat beneath it pink and glossy. Lay the slices on a warm serving platter, spoon a little sauce alongside, and bring the rest to the table in its jug. New potatoes and the first asparagus of the season belong next to this, if the calendar agrees. Tak for mad.

Chef Tips

• •Ask your butcher for a lammekulotte with the fat cap still on. Some trim it away, and without it, the crust has nothing to anchor to and the meat dries out. The fat is not waste. It's insulation, flavour, and the foundation of the skorpe.
• •Use two mustards, not one. Dijon alone is too sharp and wet. Coarse-grain alone doesn't bind. Mixed, they balance: the Dijon gives you heat and adhesion, the coarse-grain gives texture and a rounded warmth that lingers after the bite.
• •New potatoes are the only right accompaniment in spring. Boil them whole in well-salted water with a sprig of dill until a knife slides through. Toss them in butter. Nothing else. They want to taste of themselves and the season decides what goes beside this lamb.
• •If you have pan juices left from resting, pour them into the sauce. That liquid is concentrated lamb flavour and it belongs in the jug, not on the board.

Advance Preparation

• •The mustard crust mixture can be made up to a day ahead and kept covered in the fridge. Bring it to room temperature before pressing it onto the lamb, so it spreads easily without dragging.
• •Score and salt the lamb up to two hours before cooking. Leave it uncovered on a rack in the fridge. The salt draws moisture from the fat surface, which means a crisper sear when it hits the pan.
• •The sauce can be made with the pan drippings while the lamb rests, so there is no need for advance preparation on that front. Everything converges at the right moment.

Frequently Asked Questions

Nutrition Information

1 serving (about 240g)