Preheat oven to 375 F.
In a large skillet (a Dutch oven works great), fry the bacon over medium-high heat until just crispy. Remove to a paper towel-lined plate.
Remove all but 2 tablespoons of the oil left in the skillet.
Add the butter (careful, or it'll splatter) and melt.
Add the Brussels sprouts and season generously with salt and pepper.
Cook until the sprouts brown slightly along the edges, stirring occasionally.
Add the garlic and reduce the heat to low. Cook, stirring, for 1 minutes.
Add the cream and stir. Simmer, stirring for 3-4 minutes.
Make a slurry by stirring together the cornstarch and batter. Slowly stir it into the skillet. Bring the mixture to a simmer, stirring, until it's the desired thickness.
Crumble the bacon into the skillet.
Spray a 9" x 9" pan with nonstick spray. Pour in the Brussels sprouts and spread out evenly.
Top with the cheeses.
Bake 15 minutes or until hot and bubbly.