Preheat Oven to 450 degrees .
Brown the breakfast sausage in a skillet over medium heat. While the breakfast sausage is browning, chop the ends of the brussells sprouts off and place them in the food processor to shred them. If you do not have a shredding blade, slice them.
After the sausage is brown, spoon the sausage out of the pan leaving the drippings. Take the sliced onions and carmelize them in the drippings (about 5-7 minutes).
In a saute pan, melt the butter over medium heat. Once the butter is melted, poor the cream into the pan and slowly stir in the parmesan cheese. Continue to stir until the sauce is thick and smooth(3-5 minutes).
Spoon the white sauce on the flatbread and top with the sausage, brussells sprouts and onions. Bake in the oven for 10 - 12 minutes or until edges are crisp and brown.