Pat the chicken dry and season generously on both sides with salt, black pepper, and paprika.
Heat olive oil in a large skillet over medium heat. Place the chicken skin-side down and cook undisturbed for about 15 minutes, until the skin is deep golden and crisp.
Flip the chicken and add the sliced onions to the pan. Cook another 15 minutes, stirring the onions occasionally, until soft and lightly caramelized. Once the chicken is cooked through, remove it and set aside.
To the same skillet, add chicken stock, heavy cream, Dijon mustard, and whole-grain mustard. Stir, scraping up browned bits, and simmer about a minute until slightly thickened and glossy.
Return the chicken to the pan and spoon the creamy onion sauce over the top. Warm through for a minute or two. Finish with fresh parsley.
Notes
Serve with fluffy rice, mashed or roasted potatoes, crusty bread, or a simple green salad. Keep the heat gentle once the cream goes in.