Preheat your oven to 350 degrees. Line a cookie sheet with parchment paper and spray with oil.

Heat the olive oil over medium heat. Add the onion and Italian sausage. Cook until the onion is tender and the sausage is nicely browned. Remove from heat and allow to cool slightly.

Bring a large pot of salted water to a boil. Cook the pasta for 9 minutes. Carefully remove and set on the parchment lined cookie sheet to cool.

In a large bowl, combine the ricotta, 1 1/2 cups mozzarella, 1/2 cup Parmesan, parsley, salt and pepper, and garlic. Mix in the meat mixture and stir until combined.

Grease a 9x13 inch baking dish. Spoon half of the marinara sauce into the bottom of the baking dish.

Place the shells into the baking dish and stuff with the meat and cheese mixture. Spoon the remainder of the marinara over the shells and top with the remaining mozzarella and Parmesan cheeses. Top with butter pieces.

Bake uncovered for 30-35 minutes.

TO FREEZE: Prepare the casserole, but do not bake. Cover tightly with 2-3 layers of foil and label. Freeze for up to 6 months. Defrost overnight and bake as directed or bake from frozen, adding 15-20 minutes baking time, until hot and bubbly.