Polenta Pizza

1 tablespoon olive oil

1 Spanish onion, thinly sliced

1 green bell pepper, thinly sliced

1 (3.5 ounce) link hot Italian sausage, sliced

¼ cup sliced fresh mushrooms, or more to taste

1 slice prepared polenta, cut into 4x4-inch piece

¼ cup spaghetti sauce, or as needed

1 ounce shredded mozzarella cheese

Heat olive oil on a griddle or large skillet over medium heat; cook and stir onion, green bell pepper, sausage, and mushrooms until sausage is fully cooked and onions are lightly browned, 10 to 15 minutes. Remove from griddle.

Place polenta on the same hot griddle; cook until warmed through, about 5 minutes per side. Layer polenta with vegetable-sausage mixture, spaghetti sauce, and mozzarella cheese. Cook on griddle until cheese is melted, 5 to 10 minutes.