Add mustard, olive oil, thyme, rosemary, garlic, black pepper, and salt. Mix.
Season potatoes in pan with olive oil, salt, black pepper, Herb de Provence. Toss.
Score lamb fat with knife. Or poke if unable to score because of twine.
Rub seasoning into scored lamb.
Optional: wrap with kitchen twine.
Place on top of potatoes (and vegetables)
Pre-heated 350 deg F for 1-1.5 hours. Check temp after 1 hour and then every 30 mins until temp is 145 deg F
Let meat rest 20 minutes before carving.