Preheat the oven to 375 degrees Fahrenheit (190 degrees Celsius). Place a sheet pan or baking sheet in the oven to preheat as well.
Take each slice of bread, and use the back of a spoon or fingers to make an indentation in the center of each slice, creating a well for the toppings.
6-8 slices bread
Brush the tops of the bread slices with a little olive oil.
Olive oil
Spread the marinara sauce evenly over each slice of bread, making sure to stay within the indentation and leaving a small border around the edges for the crust.
1 cup marinara sauce
Distribute the sliced mushrooms, diced green pepper, and diced onion evenly over the sauce-filled indentations.
1 cup sliced mushrooms, 1/2 cup green bell pepper, 1/2 cup yellow or white onion
Sprinkle the shredded Mozzarella cheese and grated Parmesan cheese evenly over the top of the vegetables and sauce. Season with salt and pepper to taste.
1 cup shredded Mozzarella cheese, 1/2 cup grated Parmesan cheese, Salt to taste, Black pepper to taste
Place the sheet pan back in the preheated oven and bake for about 10-15 minutes or until the cheese is melted and bubbly, and the edges of the bread are crispy and golden.
Once the pizza is done, remove it from the oven and let it cool for a minute or two.
If desired, sprinkle some red pepper flakes, dried oregano, or fresh basil leaves on top for added flavor and garnish.
Optional: red pepper flakes
Use a sharp knife or pizza cutter to slice the sheet pan pizza into 6-8 pieces.
Serve the delicious sheet pan pizza slices hot, and enjoy!