Preheat oven to 400. Spray a large casserole dish with cooking spray. Put the sausage in the baking dish (cut into bite-sized pieces, if desired). Bake for 30 minutes.
Meanwhile, stir together the cream cheese, pesto, and heavy cream.
Spread the sauce on top of the sausage. Top with mozzarella. Bake for an additional 10 minutes or until the sausage is 160 degrees when checked with a meat thermometer.
Optional: Broil for 2-3 minutes to toast the cheese on top. Watch it constantly. It can burn easily.