Extra-virgin olive oil
3 pounds Yukon gold potatoes, cut into 1-inch pieces
3 pounds fennel bulb, cut into wedges through the root
1 head garlic, broken into cloves
Kosher salt and freshly ground black pepper
1 (5 to 7-pound) leg of lamb, boned
4 cloves garlic, minced
1 tablespoon freshly chopped thyme leaves
1-ounce dried porcini mushrooms, ground in a spice grinder