This is a delicious excuse to learn to carve bone-in leg of lamb. For an instant side dish, toss some potatoes around the lamb while it’s roasting. When it’s time to carve, start on the side with the crust. If some falls off as you slice, that’s okay—just scoop it onto each serving.
Preheat the oven to 375°F. In a food processor, combine the bread crumbs, pine nuts, rosemary, anchovy, garlic, and fennel seeds. Process until the nuts and garlic are finely chopped, 10 to 15 seconds. Add 1⁄2 cup of the olive oil, mustard, a pinch of salt, and a few grinds of pepper to the food processor. Process until combined.
Heat the remaining 3 tablespoons of oil in a large, heavy-duty roasting pan set over medium heat. Sear the lamb on all sides until deeply browned, 3 to 4 minutes per side. (You'll need to prop up and support the leg with tongs to sear some areas.)
Set the leg in the pan with the flatter side down. Spread the bread crumb mixture evenly on top of the leg and partially down the sides. Roast until medium rare, 70 to 80 minutes, when an instant-read thermometer inserted into the deepest section of meat (without touching the bone) should read 130°F. Let the meat rest for at least 20 minutes before carving.
