In a large skillet, bring the butter over medium heat until melted. Add the Brussels sprouts, onion and salt and stir to coat. Cook until Brussels sprouts are golden, stirring occasionally (about 8 minutes).
Add the apple and garlic and reduce the heat to low. Stir to combine and cover with a lid.
Cook until apples are tender, removing the lid occasionally to stir (6-10 minutes). Stir in the bacon pieces or sprinkle them over the top to serve.