Remove the bacon pieces, reserving the grease in the skillet.
Add mushrooms with onion, cook until onions are tender and mushrooms browned, lightly season with salt and pepper. Remove from the skillet.
Add the Brussels Sprouts and water to the skillet, season with salt and pepper. Cook over medium heat until sproutes are tender and water evaporates, about 10-12 minutes.
Stir in the garlic and butter.
Return the mushrooms and bacon to the skillet. Cook until heated.