Preparation

Prepare & cook the lamb

• In a large non-reactive dish, mix 2 Tbs. oil with the wine, mustard, garlic, tarragon, 2 Tbs. salt, and 1 tsp. black pepper. Add the lamb and coat with the marinade, cover, and refrigerate, flipping occasionally, for at least 8 hours and up to a day.
• Position a rack in the center of the oven and heat the oven to 450°F. Put the lamb on a large cutting board and cut away any fatty patches. Set the lamb in a large roasting pan. Brush with the remaining Tbs. oil and sprinkle with pepper.
• Roast the lamb until it starts to brown on top, 12 to 15 minutes. Reduce the oven’s heat to 325°F and roast until an instant-read thermometer inserted into a thicker part of the lamb registers 135°F for medium-rare, 30 to 35 minutes. Transfer the lamb to a cutting board, tent with foil, and let rest for 10 minutes.

Make the sauce

• In a medium saucepan over medium heat, heat the oil with the garlic, stirring, until the garlic sizzles steadily and just starts to turn golden at the edges, about 2 minutes. Remove from the heat and add 3/4 cup water and the vinegar. Whisk in the sugar until dissolved. Stir in 1/2 tsp. pepper. Taste and season the sauce with more sugar, salt, and pepper if needed. Let sit at room temperature while the lamb roasts.

To serve

• Stir the mint into the sauce right before serving. Thinly slice the lamb and serve with the mint sauce on the side.