Place water into a pot on the stove to boil. Add in the sweet potato and cook until fork tender (or they can be grilled)
Place fish in a non-reactive bowl and add 3 cups of chilled water to the fish and rinse gently. Drain water.
Add onions to soak in remaining 1 cup of chilled water and set aside.
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 and pepper and cilantro. Let marinade for at least 10 minutes.
Meanwhile toast the corn kernels on the stove in a tablespoon of oil. They will pop slightly when they are toasted. Place on a paper towel to remove excess oil and add salt to taste.
On a platter, place the lettuce leaves on the bottom, top with the ceviche,(reserve the marinade) add on the onion slices and serve with the sweet potatoes and corn kernels.
Serve the marinade in a shot glass along with the ceviche, in Peru it is called leche de Tigre and is believed to be an aphrodisiac and a hangover cure.