Pasta Bake

Bring a large pot of lightly salted boil water to a boil. Cook mostaccioli pasta in boiling water for 8 to 10 minutes, or until al dente. Drain well.
Meanwhile, cook ground beef and chopped onions in a skillet over medium heat until browned.
In a large bowl, mix together the mushrooms, spaghetti sauce, shredded mozzarella cheese, pasta, and browned ground beef and onion mixture. Transfer to a greased, 9x13 inch casserole dish.
Bake at 325 degrees F (165 degrees C) for 20 minutes, or until very hot.