Portabello Mushroom Black Bean Burger

Name: Portabello Mushroom Black Bean Burger
Prep time: 25 min
Cook time: 20 min
Total time: 45 min
Yield: 4
Ingredients: Ingredients; 2 large portobello mushroom caps, stems removed and gills scraped out; 1 can (15 oz) black beans, drained and rinsed; 1/2 cup rolled oats; 1/4 cup red onion, finely diced; 2 cloves garlic, minced; 1 tablespoon tomato paste; 1 teaspoon ground cumin; 1/2 teaspoon smoked paprika; 1/2 teaspoon chili powder; 1/4 teaspoon black pepper; 1/2 teaspoon salt; 2 tablespoons olive oil, divided; 4 gluten-free burger buns; 4 lettuce leaves; 1 large tomato, sliced; 1/2 red onion, sliced into rings; 1 avocado, sliced (optional); Marinade; 2 tablespoons balsamic vinegar; 1 tablespoon olive oil; 1 teaspoon dried oregano; 1/2 teaspoon garlic powder; Salt and pepper to taste