Combine honey, 3 tablespoons (45 mL) of the oil, lemon juice, mustard, onion, garlic, rosemary, salt, and pepper in a freezer zip top bag. Add lamb, turning to coat. Refrigerate overnight or up to 24 hours. Remove lamb from marinade and discard remaining marinade.
Preheat oven to 450 degrees (230 degrees Celsius).
Pat meat dry with paper towel. Heat remaining oil in oven-proof grill pan over high heat. Sear lamb for 5 minutes, or until well marked. Flip lamb, and transfer pan to oven (or use baking sheet if grill pan is not oven-proof), and cook for 20-25 minutes, to 160 degrees (70 degrees Celsius) for medium. Let rest 10 minutes before slicing.
Heat grill to high. Sear lamb for 5 minutes, and flip. Reduce heat to medium and cook for about 30 minutes or until 160 degrees (70 Celsius) for medium. Let rest 10 minutes before slicing.