Preheat the oven to 400 degrees. Heat oil in a large cast iron skillet over medium high heat.
Wash and pat dry the chicken thighs. Trim any excess skin, but not too much because it will shrink when cooked. Put the salt, paprika, garlic and dry mustard in a small bowl and mix well. Season chicken with mix.
Place the thighs, skin side down in the skillet and brown. About 3-5 minutes.
Remove chicken from skillet and set aside. Lower the heat to medium and add the wine, scraping the bits from the bottom of the pan and cook for 3 minutes. Add the broth and the mustard and bring to a boil. Cook another few minutes. Pour the sauce into a large heat proof measuring cup or bowl.
Add your sliced onions, potatoes and garlic to the pan. Top with chicken thighs. Add another dollop of grainy mustard to the top of each chicken thigh (about 1/2 tsp), then pour sauce over it all and place the skillet in the oven. Cook for 30 minutes.
Optional : set oven to broil for a minute or two at the end to get the skin a little more crispy. Watch closely so you don’t burn.