Preheat oven to 200°F.
Heat butter and olive oil in a skillet over medium heat. Brown the chicken on both sides (about 5 minutes per side). Transfer to a warm oven.
In the same skillet, sauté shallots and garlic until soft. Add wine, deglaze the pan, and let it bubble.
Stir in cream, mustard, Worcestershire sauce, and nutmeg. Simmer until the sauce thickens.
Serve chicken topped with sauce, optionally sprinkled with Gruyère. Pair with boiled potatoes, rice, or vegetables.