First, slice and soak the red onion in ice water. They will need to soak for about 30 minutes.
Next, juice between 8-16 limes, depending on their juiciness and size (psst. No waste here! Have a spectacular margarita by also making some oleo saccharum). Add the juice and the fish stock to the blender along with the garlic cloves, whole shallots, and salt. Next, rough chop the celery ribs (so they can fit into the blender) and chop about 1/4in (.3cm) of the stems at the bottom of a bunch of cilantro. Add these to the blender and proceed to the last step.
Using the edge of a spoon, peel the skin off a medium finger of ginger so that the yellow meat is showing. Grate the yellow part of the ginger with a microplane or the fine side of a box grater. Scoop up the pulp and add that to the blender.
Finish the leche de tigre by blending until smooth.
At this point you can leave the leche de tigre as is, or you can add some spice. This recipe mixes in 2 tablespoons (30ml) of ají amarillo paste, but I suggest you add to taste. (See the FAQ section for using other forms of ají amarillo peppers and different ways to spice leche de tigre)
Now that the leche de tigre is finished, place it in the refrigerator to chill and take your fish out of the fridge. Place the filet with what would have been the skin side down on the board. Slice and cube the fish into 3/4in (1.9cm) pieces. If you can, each slice should be done in one stroke, pulling the knife towards you. If you run out of knife, stop realign and pull towards you again. (do not saw at the fish)
Once cubed, gather your toppings and take the leche de tigre out of your refrigerator. Submerge the fish slices into the leche de tigre marinade and let sit for about 30-60 seconds. Using a slotted spoon or tongs move the fish from the marinade to your serving plate and top with cilantro, red onion, and as much marinade as you’d like.