Preheat your oven to 350°F (175°C) and coat a 9x13 inch baking dish with cooking spray.

In a large skillet over medium heat, heat olive oil and sauté crumbled Italian sausage for 3-4 minutes until browned. Add diced onion and cook until translucent, about 4-5 minutes, then stir in minced garlic for 30 seconds.

Transfer the sausage mixture to a mixing bowl. Add ricotta, beaten egg, mozzarella, Parmesan, Italian seasoning, red pepper flakes, salt, and pepper. Mix until creamy.

Boil a large pot of salted water and cook manicotti shells 1-2 minutes less than package directions. Drain and cool on a towel.

Spread about 1 cup of marinara sauce on the bottom of the baking dish. Fill each manicotti shell with the cheese and sausage mixture, arrange in the dish, then top with more marinara.

Cover with aluminum foil and bake for 20 minutes. Remove the foil and sprinkle on remaining mozzarella, then bake uncovered for another 10-15 minutes until bubbly and golden.

Let the manicotti rest for 3-5 minutes, garnish with fresh parsley, and serve hot.