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 water.
Add onions 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.