Boil mafalda pasta according to package directions.

Preheat oven to 350°F (175°C).

Break up Steak-Ums into small pieces with hands.

Place steak pieces in a large skillet with chopped onion, green pepper, and sliced mushrooms.

Cook over med-high heat, stirring often and chopping meat a bit more, until meat is browned and peppers are tender.

Drain excess grease from the skillet.

In a large bowl, combine the browned meat/veggie mixture with cooked and drained pasta.

Add salt, pepper, and oregano.

Stir to combine.

Add 4 cups of marinara sauce and the cheddar cheese.

Stir well until everything is evenly mixed.

Pour the mixture into a 13" x 9" baking dish.

Cut 6 slices of American cheese in half, creating 12 half slices.

Arrange the 12 half slices of cheese evenly on top of the pasta bake.

Bake at 350°F (175°C) for 25-30 minutes, or until bubbly and cheese is melted and slightly browned.