1. For the lamb: Adjust oven rack to lower-middle position and heat oven to 450 F. Using paring knife, cut 1-inch deep slits all over lamb and poke garlic slivers and rosemary leaves inside slits. Rub lamb with oil and season with salt and pepper. Place leg meaty side down on wire rack set inside rimmed baking sheet. Roast lamb for 15 minutes, flip and roast 15 minutes longer.
2. For the crust: Meanwhile, pulse bread, garlic, oil and parsley in food processor until coarsely ground. You will have 2 cups crumb mixture leftover. Stir rosemary into remaining crumbs.
3. Lower oven temperature to 325 F. Remove lamb from oven and brush meaty side with mustard. Press bread crumb mixture into mustard and continue roasting until temperature registers 130 to 135 F for medium, 30 to 45 minutes. Transfer lamb to cutting board and tent loosely with foil. Let rest 15 to 20 minutes. Slice and serve.
Make Ahead: Make slits in lamb, fill with garlic slivers and rosemary, wrap tightly in plastic, and refrigerate for up to 2 days. The garlic-herb crust may be prepared as directed in Step 2 and refrigerated for up to 1 day.