Fish: Lightly drizzle with olive oil on both sides. Cut ~6 slits on each side of the fish without going through the bone. Season with salt and pepper. Put lime slices in the cavity. Cook 9-10 minutes per side.
Add the black beans, cumin, chilli powder, and ½ cup of water to a pan. Simmer until heated through. Turn off the heat. Add the corn, mix well, and cover until ready to serve. Optional: garnish with cilantro.
In the meantime, heat 2 tablespoon olive oil in a pan. Add the peppers and onions. Season with salt and pepper. Add the lime juice. Cook until onions are translucent, ~8 minutes.
Heat your corn tortillas before serving. I heat them over open flame on my gas stove, rotating them occasionally, but you can heat them in the microwave.
Serve everything separately and let everyone create their own tacos. Don't forget to put out hot sauce and lime wedges!