In a non-metallic bowl, combine the diced fish or seafood with the freshly squeezed lime and lemon juice. Make sure the fish is fully submerged in the juice. Cover the bowl and refrigerate for about 30 minutes to 1 hour, or until the fish is "cooked" and opaque.
While the fish is marinating, prepare the other ingredients. Slice the red onion thinly and soak it in cold water for about 10 minutes to reduce its sharpness. Drain well before using.
Dice the tomatoes, finely chop the jalapeño or serrano peppers, and chop the cilantro.
After the fish has marinated, drain off most of the excess citrus juice. You can leave a little juice for added flavor if desired.
Add the sliced red onion, diced tomatoes, chopped peppers, and cilantro to the fish. Season with salt to taste and gently toss to combine.
Taste the ceviche and adjust seasoning as needed. You can also add additional lime or lemon juice if desired.
Optional: Serve the ceviche with slices of avocado, diced cucumber, or diced mango or pineapple for added flavor and texture.
Serve the ceviche chilled, either as an appetizer with tortilla chips or as a light main course.