Black Bean Burger with Mushroom Bun

Name: Black Bean Burger with Mushroom Bun
Ingredients: 4 large portobello mushrooms, about 5-inches around; 6 tablespoons olive oil; Kosher salt and freshly ground black pepper; 2/3 cup old-fashioned rolled oats; 1/2 cup roughly chopped red bell pepper (about 1/2 of a small bell pepper); 1/3 cup roughly chopped red onion (about 1/4 of a small onion); 1 large clove garlic, smashed; 1/2 to 1 teaspoon chili powder; 1/2 teaspoon smoked paprika; One 15-ounce can black beans, drained and rinsed; Ketchup, for topping; 4 small leaves butter lettuce; 4 slices vine-ripe tomatoes; 8 to 12 thin rounds red onion