Crumble the sausages into a frypan over medium-high heat. Cook until evenly browned. Add onion and cook for a few minutes. Drain excess grease and stir in the Alfredo sauce. Spoon this mixture over the pre-baked pizza base. Sprinkle cheese over the top.
Bake for 10 to 12 minutes in the preheated oven until the cheese is melted. Allow to cool slightly before cutting to allow the toppings to set.