Preheat the oven to 180° C. Grease a casserole dish.
Melt butter in a heavy bottom sauce pan over medium heat, add flour and whisk to combine. Add milk gradually and continue to whisk until mixture comes to a boil.
Season with nutmeg, salt and pepper, reduce heat to low and keep cooking stirring frequently for approximately 7-8 minutes. Check for seasoning and add more salt if necessary.
In the meantime heat the tomato sauce in a large skillet, and cook the Tortelloni according to the package instructions. Drain Tortelloni reserving a ladleful of the pasta cooking water. Add the pasta to the skillet and stir gently, adding some of the pasta water if you need to loosen the sauce. Season with salt and pepper to taste.
Put Tortelloni into the casserole dish and top with béchamel sauce. Sprinkle with grated Parmigiano cheese and bake until bubbly for about 20 minutes. Turn the oven up and broil the dish for 1 minute to brown the top if desired.