Boil mafalda pasta according to package directions.
While pasta is cooking, Preheat oven to 350. Then, break up Steak-Ums into small pieces with hands and place them in a large skillet with onion, pepper and mushroom. Cook over med-high heat, stirring often and chopping meat a bit more, until meat is browned and peppers are tender. Drain.
In a large bowl, combine the browned meat/veggie mixture with cooked and drained pasta. Add salt, pepper, and oregano. Stir.
Add 4 cups sauce and the cheddar cheese and stir well. Pour into at least a 13" x 9" baking dish.
Cut 6 slices of American cheese in half so you have 12 half slices. Arrange 12 half slices of cheese evenly on top.