Heat your oven to 375 degrees F. Grease a 9x13 inch baking dish with cooking spray or a light coating of olive oil and set it aside.

In a large skillet over medium-high heat, brown the Italian sausage, breaking it into small crumbles as it cooks. Cook for 6 to 8 minutes until it is fully browned with no pink remaining and the crumbles are nicely caramelized in spots. Add the minced garlic and cook for 1 minute, stirring constantly, until fragrant. Remove the skillet from the heat. Drain off any excess grease if the sausage is very fatty, leaving just a thin coating in the pan.

Add the marinara sauce, drained diced tomatoes, Italian seasoning, red pepper flakes if using, salt, and pepper directly to the skillet with the sausage. Stir until everything is evenly combined. Taste the sauce and adjust salt. It should taste well seasoned because the tortellini and cheeses are mild.

In a small bowl, stir together the ricotta, egg, chopped parsley, a pinch of salt, and 1/4 cup of the Parmesan. The egg helps the ricotta set slightly during baking so the dollops stay creamy but do not dissolve into the sauce.

Spread about half of the sausage-marinara mixture in an even layer across the bottom of the prepared baking dish. Scatter all of the refrigerated tortellini evenly over the sauce layer, do not pre-cook them, they cook right in the oven. Pour the remaining sausage-marinara sauce over the tortellini, making sure the pasta is well covered and coated. Drop spoonfuls of the ricotta mixture evenly across the top of the sauce. Scatter 1 cup of the shredded mozzarella and the remaining 1/2 cup of Parmesan over everything.

Cover the baking dish tightly with aluminum foil. Bake covered for 25 minutes, then carefully remove the foil. Scatter the remaining 1 cup of mozzarella evenly over the top. Return the dish to the oven uncovered and bake for another 12 to 15 minutes, until the cheese is fully melted, bubbling, and golden brown in spots. The sauce should be visibly bubbling around the edges of the pan. A tortellini pressed with a spoon near the edge of the dish should feel soft all the way through with no firm center.

Let the casserole rest for 5 to 8 minutes before scooping and serving. This short rest lets the sauce tighten up just enough so each serving holds together on the plate rather than running. Serve with crusty bread to soak up the extra sauce.