Preheat oven to 400 degrees.
Rinse Brussels Sprouts and add them to a large microwave safe bowl and cover with a wet paper towel.
Microwave for 4-5 minutes or until crisp and tender.
To a large oven-safe skillet add the oil on medium-high heat.
Add the bacon and cook until crisp.
Add the garlic, onions and Brussels sprouts and stir well.
Season with salt and pepper and cook for 5-6 minutes until Brussels Sprouts are browned and crispy.
Add butter and flour and stir until melted and combined.
Stir in the milk slowly until smooth and let simmer until slightly thickened, about 3-4 minutes.
Top with Swiss cheese and bake for 15-18 minutes.