Combine the olive oil, mustard, garlic, rosemary and pepper to form a paste. Brush the mixture all over the rack of lamb. Let rest in refrigerator for at least 1 hour. Remove from refrigerator 15 minutes before roasting. Preheat oven to 375°F.
In a small heavy skillet, heat the remaining 2 tablespoons of olive oil over high heat. When the oil is very hot, brown the lamb, forming a crust, about 5 minutes per side.
Rapidly spread the bread crumbs over the top of the rack and drizzle with the melted butter.
Immediately place the hot pan in the oven. Roast until an instant-read thermometer inserted in the thickest part reads 135°F, about 12 minutes.
Transfer the rack of lamb to a serving platter or carving board. Let rest 5 to 10 minutes before slicing. Slice in between the bones, forming either single or double chops.