Boil sweet potato and corn (separately) if using for garnish. Allow to cool. (Can be done hours or even a day in advance)
Wash and trim your fish. Slice into pieces between ½ inch (15 mm) cubes to 2 inch (50mm) pieces, depending on taste.
Place fish in a non-reactive, shallow pan in a thin layer. Season with salt and pepper.
Combine lime juice, chili pepper, coriander and garlic. Pour mixture over fish. Stir lightly to expose all the fish to some of the lime juice mixture.
Put sliced onion on top of fish as it “cooks”
Let fish stand for 10 minutes.*** Lift fish out of the lime juice and plate individual portions , garnishing with lettuce, slices of sweet potato and slices or kernels of corn if using.