Meanwhile, in a large skillet over medium-high heat, sauté the mushrooms in the butter until golden. Season with salt and pepper. Add the onions and garlic. Continue cooking for 2 minutes, stirring constantly. Deglaze with the vodka and flambé, if desired. Add the béchamel, tomatoes and pasta. Stir well to coat the pasta in the sauce. Adjust the seasoning. Transfer to a large serving bowl. Sprinkle with Parmesan cheese and green onions.