In a saucepan, drizzle the olive oil and add the minced onion and garlic. Cook for a minute, then add the tomato puree.
Bring to a simmer and turn down the heat. Season with salt, and black pepper and add a handful of basil leaves. Simmer for at least 15 minutes. It will turn even better if you can simmer it for 30-40 minutes.
In a large pot of boiling salted water, cook the tortellini for half the time than what's recommended on the package. If you use fresh or frozen tortellini, you can skip this step and add them directly into the baking dish with the sauce.
In a baking dish, spread a thin layer of white sauce at the bottom, then add some tomato sauce, half of the tortellini, the mozzarella cut into slices or shredded with your hands, and some grated Parmigiano.
Repeat again the layers with tomato sauce, white sauce, and the remaining tortellini. Finish with the remaining tomato sauce and white sauce on top, covering well the tortellini so they don't dry out. If you want, you can mix the tortellini with the sauces with a spoon.
Sprinkle abundant grated Parmigiano on top.
Bake in the preheated oven at 400°F/200°C for about 15-20 minutes. Turn on the broiler for the last 3-4 minutes to get a nice golden crust on top.
Notes
Tortellini: You can use dried, fresh (refrigerated), or frozen tortellini for this recipe. Only the dried ones need to be boiled for a few minutes, while the fresh and frozen ones can be added directly to the baking dish since they have a shorter cooking time.
Béchamel Sauce: Also called Besciamella or White Sauce. You can find it already made in jars or packages or make your own using this recipe to make Béchamel Sauce.
Tomato Sauce: You can also use store-bought marinara sauce to make this recipe in a shorter time.
You can store leftovers in an airtight container in the fridge for up to 3 days. Reheat in the microwave.