Preheat oven to 400°F. In a small bowl whisk together the mustard, oil, garlic and rosemary. Set aside. Wash, quarter then quarter again each potato.
In a large oven roof skillet (like cast iron) over medium-high heat add butter and cook potatoes for 8-10 minutes until golden brown on edges. Season with salt and pepper then transfer to oven to roast for 30 minutes.
Remove pan from oven and add mustard sauce to roasted potatoes. Stir to coat evenly and cook on medium-high heat for 5 minutes on stovetop. Serve with chopped parsley or more rosemary if desired.