Al Roker's Veggie Burger with black beans and mushrooms

Ingredients

• 2 tablespoons olive oil, divided
• 1/2 cup white onion, diced
• 3 cloves garlic, minced
• 2 cans black beans, drained
• 10 medium button mushrooms, stems removed
• 1 teaspoon cumin
• 1 teaspoon salt
• 1/2 teaspoon pepper
• 1 egg
• 1/2 cup cooked brown rice
• 1 cup whole wheat bread crumbs
• 1/4 cup Sriracha
• 1/2 cup mayonnaise
• 4 whole wheat buns
• 6 leaves Bibb lettuce
• 1 tomato, sliced
• 1 avocado, sliced