1) Pre-grease the barbecue and heat to medium-high. Place the fish in a non-reactive bowl, add 3 cups of chilled water and rinse the fish gently. Drain the water. Add the onions to the remaining chilled water and allow to soak.
2) Meanwhile, place the aji, garlic and a pinch of salt in a mortar and pestle. Grind to make a paste.
3) Combine the fish, lime and lemon juices, aji and garlic paste, salt, pepper, and coriander. Leave to marinate for 10 minutes.
4) Brush the corn and sweet potato slices with oil and place on the grill. Cook for about 10 minutes.
5) When ready to serve, divide the 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.