In a medium bowl, mash the black beans with a fork until they are partially broken down.
Stir in the breadcrumbs, onion, garlic, smoked paprika, and cumin. Season with salt and pepper.
Form the mixture into four patties.
Brush the portobello mushrooms with olive oil and season with salt and pepper. Grill or bake for 5-7 minutes per side, or until tender.
Heat a lightly oiled skillet over medium heat. Cook the black bean patties for 4-5 minutes per side, or until golden brown and cooked through.
Serve the black bean burgers on the portobello mushroom buns with your favorite toppings.