Preheat oven to 350 degrees. Spray an 8x13 baking dish lightly with non-stick spray.
Set aside approximately 24 pepperoni slices. Cut the remaining pepperoni into quarters.
Place the precooked pasta in the baking dish. Pour the crushed tomatoes on top. Add the pepperoni pieces, drained mushrooms, and Italian seasoning. Stir together until the pasta is well coated with the crushed tomatoes.
Top evenly with the shredded cheese. Add the pepperoni slices. Bake for 30 minutes until the cheese is melted, golden and bubbly.