Melt the butter in a large ovenproof skillet over a medium heat. Thinly slice the onion. Add the onion to the skillet and sweat gently for 4-5 minutes until translucent and golden. Remove from the skillet with a slotted spoon and set to one side.
Add the chicken thighs to the skillet used to cook the onion, skin side down. Leave to cook for 4-5 minutes until the skin is crisp and golden. Turn the chicken pieces over and cook for a further 4 minutes or until browned all over.
Return the onion to the skillet, arranging it around the chicken thighs. Add the fresh thyme. Let the onion and thyme cook for a few minutes in the chicken fat until the onion starts to caramelize.
Add the chicken stock and balsamic vinegar to the skillet. Stir gently to combine. Bring to a boil then reduce to a simmer. Cover and simmer for 12-15 minutes or until the stock has reduced down and the chicken thighs are completely cooked through.
Preheat a grill to high. Roughly chop the Camembert and divide between the four chicken thighs, covering the tops of each. Place the skillet under the grill for 4-5 minutes or until the cheese is bubbling and melted. You may use an alternative cheese if preferred. Serve the dish hot with your preferred Keto sides.