Heat oven to 450 degrees Fahrenheit. Prepare a rimmed sheet pan with cooking spray.
Place brussels sprouts cut side down on the large baking sheet in a single layer.
Sprinkle chopped, uncooked bacon and diced onions on baking sheet.
Drizzle with balsamic vinegar and olive oil. Sprinkle with salt and pepper.
Roast for 20-25 minutes in the preheated oven until caramelized and outer leaves are starting to turn a deep golden brown.
During the roasting process, remove pan from the hot oven and shake pan 1-2 times during the cooking process. Use a fork to flip any brussels sprouts back over to their flat sides if needed. m