Heat the oven to 425°F and toss the Brussels sprouts and onion with the olive oil, 1 teaspoon salt and black pepper in a 9-by-13-inch dish.
Roast the vegetables until browned at the edges, about 15 minutes.
Season the chicken pieces with the remaining salt and fold them into the hot vegetables.
Whisk the cream with the Dijon, pour it over the dish and scatter the bacon on top.
Bake until the chicken reaches at least 165°F and the sprouts are tender, 18 to 22 minutes.
Add the Gruyere and broil until browned in spots, 2 to 3 minutes.