Heat the oil in a cast iron skillet.Season the chicken thighs with salt and pepper, and place skin-side down in the skillet.Cook the chicken until the skin is brown, then turn the thighs over.Continue to cook over a medium heat until they have reached an internal temperature of 165F.

Set the chicken aside and keep warm.Remove any excess fat from the skillet, then add the onions, garlic, and thyme.Once the onions have softened, add the chicken stock and let simmer for five minutes.Add the cream and mustard, then adjust seasoning as necessary.Serve the chicken with the sauce poured over each thigh.