I start by cutting each shrimp in half lengthwise, which helps them cure faster and makes for easier eating later. I lay them in a single layer in a glass dish so the lime juice can reach every piece. A light pinch of salt at this stage helps season the shrimp from the very beginning.
Next, I squeeze fresh lime juice over the shrimp until they're fully coated. I let them sit for about 20 to 25 minutes, flipping halfway through so both sides get equal contact. I know they're ready when the shrimp turn an opaque, soft pink color, just like cooked shrimp.
While the shrimp cures, I thinly slice my red onion and toss it into a small bowl with a generous pinch of salt. I cover it with water and add a splash of white vinegar, which keeps the color vivid. This sits for the same 20 minutes the shrimp needs.
I add my jalapeños, serrano, garlic, cilantro, lime juice, and salt to a blender. I blend everything until it's completely smooth and a vibrant, almost electric green. This bold green marinade is really what makes aguachile so distinct from a classic ceviche.
Once the shrimp has cured, I pour the green chili marinade right over it, leaving the original lime juice in the dish. I stir gently so every piece gets coated. Then I drain my pickled onions and scatter most of them on top, saving a few for garnish.
I refrigerate the mixture for at least 30 minutes, though I've let it go up to a few hours with great results. Right before serving, I taste and adjust the salt, then top everything with sliced avocado, cucumber, and fresh cilantro leaves. A drizzle of olive oil is optional but always welcome.