Preheat the oven to 375° F. Grease a casserole dish. Melt the butter in a heavy bottom saucepan 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 to taste, 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 package instructions. Drain Tortelloni reserving ½ cup of the pasta cooking water. Add the pasta to the skillet and stir gently, adding pasta water a little at a time 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 Reggiano 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.