Remove sausage from casings. Add to a large saute pan over medium heat with one tablespoon of olive oil. Cook for five minutes, then add chopped onion and minced garlic. Continue cooking until onions are transparent and chicken sausage is cooked through (about another 5 minutes).
Add spinach to pan, and continue cooking until wilted. Remove from heat and set aside.
In a large bowl, combine chicken sausage mixture, ricotta cheese, and ½ cup of the mozzarella.
Spray a 9x13 inch baking pan with cooking spray. Pour about ½ cup of the alfredo sauce in the bottom of the pan. Stuff shells with 1 to 2 tablespoons of the chicken sausage mixture and set in pan.
Cover shells with remaining alfredo sauce, then sprinkle with remaining mozzarella.
Bake at 350 degrees for 15 to 20 minutes until bubbly and cheese begins to brown.