Heat olive oil in a large cast iron skillet (or oven safe skillet), over low heat. Slowly cook onions until caramelized, stirring occasionally, about 10-12 minutes.
Remove skillet from heat and add the potatoes. Sprinkle with dried rosemary, salt and pepper and toss to combine.
Bake for 20 minutes.
While potatoes are baking, coat chicken with stone mustard and set aside.
To make the sauce, mix all the sauce ingredients except the wine in a bowl. Taste and adjust accordingly. Then add in the wine.
Remove pan from oven after 20 minutes and arrange chicken over potatoes. Pour prepared sauce on top, making sure everything is coated. Top with rosemary sprigs.
Cover with foil and bake for 25-30 minutes. After 25-30 minutes, remove foil and broil for 10 minutes or until a nice brown color forms.