Brown the meat in a couple of tablespoons of oil, turning it often to color it evenly. Season with salt and pepper, then transfer it to a baking dish and bake it for half an hour at 180 °.
Pour half glass of water into the pan, put it on the heat and collect the bottom of the meat. Meanwhile, melt the butter in a small saucepan and add the chopped shallots and garlic, rosemary, salt and pepper. Brown 2-3 minutes and blend with the wine.
Add the lamb water, bring to a boil and reduce to medium heat. Strain the sauce through a sieve, put it back in the saucepan and add a teaspoon of cornstarch if it is too liquid. Salt and pepper. Remove the lamb from the oven and let it rest before slicing it.
Coat with the sauce and serve.