Preheat and prep: Heat the oven to 375°F (190°C). Grease a 9x13-inch casserole dish with a little olive oil.

Brown add-ins (optional): If using sausage, crumble it into a skillet over medium heat with a drizzle of olive oil. Cook until browned, 6–8 minutes. Add mushrooms or bell pepper if using and sauté until soft. Stir in garlic for the last 30 seconds. Season lightly with salt and pepper. Set aside.

Warm the sauce: In a large bowl, mix marinara, Italian seasoning, a pinch of red pepper flakes, and the cream or half-and-half. Taste and adjust salt and pepper. The cream softens acidity and makes the bake extra cozy.

Layer the casserole: Add the uncooked refrigerated tortellini to the casserole dish. Pour the sauce over and toss gently to coat. If using spinach, stir it in now so it wilts as it bakes. Scatter the cooked sausage or veggies evenly on top.

Add the cheeses: Dollop ricotta in small spoonfuls across the surface. Sprinkle mozzarella over everything, then finish with Parmesan for a golden top.

Bake: Cover tightly with foil and bake for 20 minutes. Remove the foil and bake another 10–15 minutes, until the cheese is melted and bubbling and the edges are lightly browned.

Rest and garnish: Let the casserole rest 5 minutes so it sets slightly. Garnish with chopped basil or parsley. Serve warm.