Start by adding the mushrooms, pepperoni, and onion to a small, non-stick skillet and bring to a medium heat. Cook for about 8 minutes or so to render some of the fat from the pepperoni. Add the mixture to a paper towel lined plate, then place the mixture into a small bowl.

Wipe out the skillet, and return to the stove. Butter each slice of the bread as you normally would when making a grilled cheese. Add one slice, butter side down into the skillet, and bring to a medium-low heat. Spread half of the warmed marinara sauce onto the bread, then top with the pepperoni mixture, then top with the mozzarella cheese. Drizzle the remaining sauce on top of the cheese, then add the remaining slice of bread, butter side up. Cook slowly, allowing for the cheese to fully melt, then flip, and continue cooking until you have a nice and crispy exterior, with a hot and cheesy interior.

Plate and serve.