Preheat your oven to 400ºF (200ºC).
In a large, heavy, ovenproof skillet or Dutch oven, such as cast-iron, combine the sliced onion and chicken stock. Season the mixture with a large pinch of salt and a few grinds of black pepper. Place the chicken pieces, skin-side-up, on top of the onion mixture and season them generously with salt and pepper.
Place a butter slice on top of each piece of chicken and roast until the chicken is browned and cooked through, and registers at least 165ºF (74ºC) on a digital thermometer, 40 to 45 minutes. TIP: If your chicken is cooked through but the skin isn’t as golden as you’d like, turn on the broiler for a couple of minutes to crisp up the skin. My chicken is usually done in 40 minutes, so start checking at 30 minutes. You don't want tough chicken after browning under the broiler!) Use tongs to transfer the chicken pieces to a serving platter and cover them with foil to keep warm.
Set the skillet with the onions on the stove over high heat and bring to a boil. Let the mixture boil until it thickens slightly, 5 to 10 minutes.
Reduce the heat to low and whisk in the mustard and sour cream and season the gravy to taste with salt and pepper.
Pour the gravy mixture over the chicken pieces. Serve immediately.
