Pre-heat oven to 400F.
Place brussel sprouts in a baking dish and toss in olive oil and season generously with salt and pepper.
Roast in oven for 15-20 minutes, or until semi-softened.
Meanwhile, in a large bowl, whisk together egg, heavy cream, rosemary, garlic powder, onion powder and red pepper flakes (if using), then stir in cheeses.
Remove brussel sprouts from oven and add 3/4 of the bacon to the baking dish. Pour cheese mixture over the top and stir everything together, then top with remaining bacon.
Reduce oven temperature to 350, return baking dish to oven and bake for 20-25 minutes, or until cheese is bubbly and just set.
Remove from oven and let cool 5 minutes before serving.
