Preheat the oven to 350 degrees F (175 degrees C).
Crumble the sausage into a skillet 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 crust. 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.