Put a large pot of salted water on to boil for the shells and coat a 13 x 9-inch baking dish with nonstick spray.
Heat the olive oil in a large skillet over medium heat. Add the sausage and sauté until lightly browned, 4 to 5 minutes. Use a wooden spoon to crumble the sausage into very small pieces as it cooks.
Add the onion and garlic and continue to sauté until the onion is soft and translucent, 3 to 4 minutes longer.
Transfer the sausage mixture to a bowl and set aside for 10 minutes to cool.
While the sausage cools, cook and drain the shells according to package directions. As soon as they're cool enough to handle, arrange them in a single layer on a flat surface.
Preheat the oven to 375°F.
Add the ricotta, cream cheese, 1/2 cup of the shredded mozzarella and parsley to the sausage mixture. Combine well, then fill each shell with about 1-1/2 tablespoons of the sausage-cheese mixture.
Spoon a little of the marinara into the bottom of the prepared baking dish and spread it around as evenly as possible. Arrange the stuffed shells on top, then top with the remaining shredded mozzarella.
Bake until the cheese is bubbly and golden, 18 to 20 minutes. Serve immediately.