Saute the onion in 1 Tbsp of olive oil for 3 minutes until translucent. Add the beans, corn and taco seasoning. Cook for 5 minutes until heated through.
Combine all of the ingredients for the pico de gallo in a bowl. Set aside.
Spoon 3 Tbsp of filling into a romaine leaf. Top with the cabbage, pico de gallo, and avocado.
Serve with lime edges.