Mushroom Sliders

Ingredients (11)

• ✓ 1 pound lean ground beef, or more to taste
• ✓ 1 large egg
• + 1 small onion, finely chopped add to pantry
• + 1 cup finely chopped mushrooms add to pantry
• + 1 teaspoon ground black pepper add to pantry
• + ½ teaspoon garlic salt add to pantry
• + salt to taste add to pantry
• + 6 portobello mushrooms, or more to taste add to pantry
• + 1 green bell pepper, halved and seeded add to pantry
• + 1 red bell pepper, halved and seeded add to pantry
• + 1 yellow bell pepper, halved and seeded add to pantry