Brown ground beef, green peppers, and mushrooms in a pan until the beef is cooked through and vegetables are softened.

Drain any excess grease from the pan.

Combine the browned beef mixture with your favorite spaghetti sauce in the pan.

Simmer the sauce and beef mixture for a few minutes to allow the flavors to meld.

While the sauce simmers, cook the spaghetti according to package directions until al dente.

Drain the cooked spaghetti and rinse with cold water.

Mix the cooked spaghetti with the meat sauce mixture.

Pour half of the spaghetti and meat sauce mixture into the bottom of a 9 x 13 inch casserole dish.

Layer half of the pepperoni slices over the spaghetti mixture.

Sprinkle half of the shredded cheese over the pepperoni slices.

Pour the remaining spaghetti and meat sauce mixture over the cheese layer.

Layer the remaining pepperoni slices on top of the spaghetti mixture.

Sprinkle the remaining shredded cheese over the pepperoni slices.

Bake in a preheated 350 degrees F (175 degrees C) oven for 25 to 30 minutes, or until the cheese is melted and golden brown.

Remove the casserole dish from the oven and let it sit for at least 5 minutes before serving.