Bring the leg of lamb out of the refrigerator 4 hours before cooking it. Thinly slice the potatoes. Take 1 large teaspoon of stripped thyme, grind 2 bay leaves and finely chop the onion. Combine all these ingredients with 1 teaspoon of salt and a generous pinch of freshly ground pepper. Butter an ovenproof dish and fill with 2 layers of potatoes. Sprinkle the herb and spice mixture over each layer. Add half of the butter, cut into pieces, and add warm water to barely cover the potatoes. Preheat the oven for 10 minutes and bake on a hot oven (410 F) for 1 1/2 hours minimum. The potatoes are cooked when there is only a little liquid left at the bottom of the dish. Cream the remaining butter with the salt and pepper (3/4 teaspoon). Spread over the leg of lamb, larded with slivers of garlic. Calculate the cooking time before putting into the oven (plan on 15 to 18 minutes per lb). Rest it directly onto the potatoes. Turn the lamb over when brown. When roasted, let the meat rest for 5 minutes, oven turned off and door ajar. Serve in the ovenproof dish, slice the lamb on a board, retrieving the juice. The success of this delicious dish holds in the potatoes being completely cooked when you add the lamb.
