Spicy Vegan Portobello Mexican Burgers

portobello mushroom caps (stems removed)

grape seed oil

large onion (cut in half and then sliced into thin slices)

bell pepper (sliced into strips)

taco seasoning (divided)

cayenne pepper (divided)

whole or refried black beans with lime (divided)

buns of your choice (pretzel buns pictured)

salsa (hot)

banana peppers (to garnish)