Pat the white fish trimmings dry with paper towels and cut into 1–2 inch chunks. Thinly slice the red onion, chop the garlic and chiles, and roughly chop the cilantro. If the onion is very strong, soak the slices in cold water for 5–10 minutes, then drain and pat dry.
8 oz white fish trimmings or small pieces, 1/2 small red onion, 2 cloves garlic, 1 to 2 small fresh chiles, 1/4 cup fresh cilantro
In a high-speed blender, add the fish pieces, sliced onion, garlic, chiles, cilantro, celery (if using), salt, pepper, and ice cubes. Pour in the cold stock, lime juice, and lemon juice, adding the liquids last to help the blender pull everything down smoothly.
8 oz white fish trimmings or small pieces, 1 cup lime juice, 1/4 cup lemon juice, 1/2 small red onion, 2 cloves garlic, 1 to 2 small fresh chiles, 1/4 cup fresh cilantro, 1/2 cup fish or vegetable stock, 1 to 2 tsp kosher salt, 1/4 tsp black pepper, 2 to 3 ice cubes, 2 to 3 Tbsp celery
Blend on medium-high for 30–45 seconds, just until the mixture looks creamy, pale, and milky, like a light citrusy broth. Avoid over-blending so the mixture does not heat up; if your blender runs hot, pulse in short bursts.
Taste a small spoonful and adjust the seasoning. Add more salt if needed, more lime juice for extra acidity, or additional chopped chile or aji amarillo paste if you want more heat.
1 cup lime juice, 1 to 2 small fresh chiles, 1 to 2 tsp kosher salt, 1 to 2 tsp aji amarillo paste
Set a fine-mesh strainer over a bowl or large measuring cup. Pour the blended mixture through the strainer, pressing gently with a spoon or spatula to extract as much liquid as possible. Discard the remaining solids.
Transfer the strained leche de tigre to a bowl or jar, cover, and refrigerate for at least 30 minutes so it chills thoroughly and the flavors settle.
To make ceviche, pour the chilled leche de tigre over small cubes of very fresh fish or seafood in a glass or ceramic bowl and marinate for 10–20 minutes, depending on how “cooked” you like your ceviche. You can also spoon it as a sauce over grilled fish, shrimp, scallops, or roasted vegetables, or serve small chilled shots.