Step 1: Prepare your fish. Ensure your white fish is as fresh as possible. Pat it dry with paper towels. Using a very sharp knife, cut the fish into uniform, bite-sized cubes, about 1/2 to 3/4 inch in size. Place the diced fish in a non-reactive bowl.
Step 2: Prepare the red onion. Thinly slice half of a red onion into delicate rings. Place these slices in a bowl of ice water for about 5-10 minutes to mellow their intensity.
Step 3: Make the ‘Leche de Tigre’ base. In a blender, combine a small handful of fish scraps (optional), a tiny piece of the remaining red onion, one small garlic clove, a small piece of aji amarillo (or serrano chili), a few cilantro stems, and a splash of lime juice (from the 1.5 cups). Blend until smooth. Strain this mixture through a fine-mesh sieve into a small bowl, pressing down on the solids to extract all the flavorful liquid. Discard solids.
Step 4: Combine the fish and lime juice. Pour the freshly squeezed lime juice over the diced fish in the non-reactive bowl. Gently toss to ensure all pieces are coated. Add the strained ‘Leche de Tigre’ base to the fish.
Step 5: Add the aromatics and chili. Drain the red onion slices from the ice water and add them to the fish. Finely mince another small piece of aji amarillo or serrano pepper (adjust to your spice preference) and add it to the bowl. Stir in about two tablespoons of chopped fresh cilantro.
Step 6: Marinate and chill. Gently toss all ingredients together. Cover the bowl with plastic wrap and refrigerate for 20-30 minutes. This is the sweet spot for marinating; any longer and the fish can become tough.
Step 7: Taste and adjust. After marinating, taste a piece of fish. Adjust seasoning as needed, adding a pinch of salt if necessary. A dash of additional lime juice can brighten the flavors.
Step 8: Serve immediately. Spoon the ceviche into individual serving bowls. Garnish with additional fresh cilantro, the cooked corn kernels, and diced sweet potato (if using). Serve immediately for the freshest experience.