Clean mushrooms, remove stems and scrape out gills with a spoon. Brush with olive oil. Arrange on baking sheet. Bake 20 minutes, turning halfway through cooking.
Prepare the Black Bean Filling
Heat oil in large skillet over medium heat. Add onion and garlic. Saute until onion is softened, 5 minutes. Add beans, cumin, ancho chili powder, lime juice, salt and pepper. Cook just until beans are heated throughout, about 2 minutes.
Assemble
Divide black bean filling among portobello caps. Top with desired toppings.