Prepare the Shrimp:Place the shrimp in a bowl and cover with the juice of 4 lemons. Let it sit for 15–20 minutes or until the shrimp turns pink and opaque.
Soften the Onions:While the shrimp marinates, soak sliced red onion in a bowl with water, salt, and a splash of vinegar. Let it sit for 10 minutes to reduce sharpness.
Blend the Green Sauce:In a blender, combine chopped cucumber, jalapeño, cilantro, garlic, salt, lemon juice, and water. Blend until smooth.
Assemble the Aguachile:Drain most of the lemon juice from the shrimp, leaving just a splash. Pour the green sauce over the shrimp and stir to coat evenly.
Garnish & Serve:Top with the soaked red onion, extra diced cucumber, cilantro leaves, and sliced avocado. Serve chilled on crunchy tostadas.