Cube fish and place in a nonreactive dish.

Juice limes into a food processor. Squeeze them well, but avoid the membrane, which can cause bitterness. Add garlic and cilantro. Seed hot pepper, remove white ribs, and whack it into a few pieces; throw it into the processor. Pulse until garlic cloves and pepper are finely minced. Pour this mixture over fish. Slice onion and add it to fish.

Refrigerate and let fish marinate overnight. Stir once or twice.

Next day, drain off most of the lime juice, leaving enough to keep fish moist. Salt and pepper to taste.