1. Preheat oven to 425°.
2. Toss potatoes with oil in a shallow roasting pan. Bake 15 minutes.
3. Meanwhile, combine mustard, 1 tablespoon rosemary, thyme, and garlic; spread over meaty side and ends of lamb rack.
4. Add onion wedges, remaining 2 tablespoons rosemary, salt, and pepper to potatoes; mix well. Push mixture to edges of pan; place lamb rack, meaty side up, in center. Bake 22 minutes or until internal temperature of lamb reaches 140° for medium-rare. Transfer rack to a carving board; tent with foil, and let stand 5 minutes (temperature will rise to 145°).
5. Transfer potato mixture to 4 warm plates. Cut lamb crosswise into 8 chops; transfer to plates and serve.