Pat the chicken dry with paper towels. Season them with salt and pepper on both sides.
Heat olive oil in a large cast iron skillet over medium heat. When the oil is hot, carefully place the chicken skin side down in one layer. Let them cook for 15-20 minutes. Do not move them, let them be. Set the timer and cut the onions, if you haven't done so or sit back and relax with a glass of wine.
Turn them with tongs, cook for 10 minutes. If there's too much oil in the pan, get the chicken out to a plate. If there's too much oil, tilt the pan and scoop some out.
Add the onions to the pan and put the chicken back on top. Cook another 5-10 minutes, until the onions are browned and chicken are cooked. Stir the onions occasionally.
Transfer the chicken to a plate, leave the onions in the pan. If the onions aren't browned, cook them another minute or two.
Add the wine, crème fraîche, Dijon mustard, whole-grain mustard, salt and pepper (to taste) and stir for a minute.
Return the chicken, skin side up, get all the juices into the skillet. Sprinkle with chopped parsley and serve hot. Enjoy!
Notes
Look for coarse-ground mustard or stone-ground mustard if you can't find whole grain mustard. For crème fraîche, I think regular heavy whipping cream is a good substitute in this recipe, but your sauce will be thinner.
Recipe by Let's Cook Some Food at https://www.letscooksomefood.com/chicken-with-creamy-mustard-sauce/