Have grill pre-greased and preheated to medium-high.
Place fish in a non-reactive bowl. Add 3 cups of chilled water to fish and rinse gently. Drain
to remaining chilled water and let soak.
Meanwhile, place aji, garlic, and pinch of salt in a mortar and pestle
. Grind to make a paste.
Combine fish, lime
and lemon juices, aji and garlic paste, salt, pepper, and cilantro
. Let marinate for 10 minutes.
Brush corn and sweet potato slices with oil and place on grill
. Cook about 10 minutes.
When ready to serve, divide fish ceviche
among 4 lettuce cups and top with onion slices. Serve with grilled corn
and sweet potato
slices and garnish
with Japanese seaweed, if desired.