Place the cleaned shrimp in a shallow bowl or baking dish with 1 cup of lime juice and ½ teaspoon of kosher salt. Mix together to combine and cover. Refrigerate for 30 minutes, stirring halfway through to ensure the shrimp is evenly coated.
Make the salsa marinade by adding the serrano peppers, cilantro, and the remaining ½ cup lime juice to a blender. Blend until smooth.
Pour the marinade through a fine mesh strainer and into the bowl of shrimp. Stir to combine. Place back in the refrigerator and marinate for an additional 30 minutes.
Add the red onion, cucumber, and avocado into the bowl of shrimp and lightly toss to combine. Taste and adjust salt if needed.
Serve aguachiles with tostadas or tortilla chips.
Notes
You can adjust the spice based on your preference. If you want it less spicy, you can use jalapeños instead of serranos.
Using quality shrimp is important to make sure you get the best flavor. You can also use frozen shrimp that's been thawed.
If the idea of using raw shrimp makes you uncomfortable, you can use cooked shrimp instead but marinate for half the time. This will not make the traditional aguachile but will give you a good idea of how it tastes.
Aguachile is best served fresh. The shrimp is cured in lime juice, so the longer it stays in the lime juice, the longer it cooks. I recommend not storing it for more than 6 hours or the shrimp will continue to get tough and rubbery.