For a vegetarian option, these stuffed mushrooms are filled with a flavorful mixture of vegetables and cheese. The recipe calls for fresh mushrooms, bell peppers, onions, garlic, spinach, feta cheese, breadcrumbs, and olive oil. Start by sautéing the bell peppers, onions, and garlic in olive oil until softened. Add the spinach and cook until wilted. In a bowl, mix together the sautéed vegetables, crumbled feta cheese, breadcrumbs, and minced garlic. Stuff the mushroom caps with the mixture and bake until the mushrooms are tender and the filling is golden. These vegetarian stuffed mushrooms are a delicious and healthy appetizer choice.