To prepare the pickled onions, bring 1 quart of water to a boil in a medium saucepan.
Add onions and cook for 45 seconds.
Drain onions and rinse under cold water.
Place onions in a non-corrosive bowl and add Sugar, oregano, pepper, cumin, salt, garlic and vinegar or lime juice and stir to combine.
Add enough water to the bowl to make the pickling liquid barely cover the onions.
Cover and refrigerate for several hours, preferably overnight.
To prepare the marinade, add the Bijol, pepper, cinnamon, coriander, oregano, cloves and cumin and pulse the grinder on and off a few times to combine the spices.
Place in a small bowl and set aside.
Finely chop the garlic.
Sprinkle sea salt over garlic.
Using the flat side of a heavy chef's knife, mash and smear the garlic and salt mixture, working it back and forth on the cutting board until it forms a paste.
Add the garlic paste along with the vinegar or lime juice to the bowl with the spice mixture and mix until thoroughly combined.
Spread about a teaspoon of the marinade over each side of the Catfish fillets (there may be a little marinade left over).
Place the fillets on a sheet pan, cover with plastic wrap and refrigerate for 1 to 2 hours.
About 1 hour before serving, prepare a charcoal fire for grilling and let it burn until the coals are hot.
To grill the marinated fillets, brush them with a little olive oil and place on the grill over the hottest part of the fire.
After about 30 seconds, turn the fillets 90 degrees and grill on the same side for 30 seconds longer.
Move the fillets to a cooler part of the fire and cook for 2 minutes longer on that side.
Turn the fillets over and cook for 3 to 4 minutes on the other side or until fish flakes easily when tested with a fork.
Serve the Catfish on warm plates topped with pickled red onions and a sprig of cilantro.