1 tablespoon plus ¼ cup olive oil, divided1 large (or two small) white onions, diced2 15-ounce cans black beans (do not drain or rinse)1 tablespoon chili powder2 teaspoons dried oreganosalt and pepper to taste8 corn tortillas 2/3 cup shredded cheddar cheese1 large tomato, diced1 avocado, diced