Preheat oven to 375 degrees f. Spray a deep 9x13-inch dish with nonstick cooking spray.
Start browning the Italian Sausage over medium heat in a large skillet. When it is about halfway browned, add the onion and continue cooking until the sausage is browned and the onion is soft. Drain any excess grease. Reduce the heat to medium-low and add the spinach, stirring and cooking until wilted. Add the garlic and continue cooking for 1 minute while stirring constantly. Season with kosher salt and black pepper to taste. Set the mixture aside.
In a medium bowl, stir together the ricotta cheese, Parmesan Cheese, Italian seasoning, and egg.
Spoon ½ cup marinara into the bottom of the prepared casserole dish. Spread a layer of cheese ravioli, followed by the browned sausage mixture, then about 2 ½ cups marinara. Follow up with another layer of cheese ravioli, the ricotta mixture, another layer of cheese ravioli, about 2 ½ cups marinara, and the shredded mozzarella.
Cover with aluminum foil and bake for 1 hour. Remove the foil and bake for 20-30 minutes or until the cheese is melted and lightly browned and the casserole is heated through. Sprinkle with fresh parsley.
