Heat the oven to 400°F and grease a 9-by-13-inch baking dish.
Cook the bacon in a large oven-safe skillet over medium heat for 7 to 9 minutes, until crisp, then transfer it to a paper-towel-lined plate.
Add the Brussels sprouts to 2 tablespoons bacon fat and cook cut-side down for 6 minutes, until browned.
Boil the gnocchi for 2 minutes or until they float, then drain them well.
Stir the garlic, cream, mustard, salt and pepper into the sprouts and simmer for 2 minutes.
Fold in the gnocchi, bacon and mozzarella, transfer to the baking dish and top with Parmesan.
Bake for 15 to 20 minutes, until browned on top and bubbling around the edges.
Rest for 5 minutes before serving.