Marinate the shrimp in lime juice, salt, cumin, and pepper. Make sure to have enough lime juice to cover the shrimp. Refrigerate for 20-30 minutes and mix occasionally to ensure the shrimp is evenly coated with lime juice. This helps “cook” it. You will see how the shrimp turns into a light pink color. If you have trouble covering shrimp in lime juice try using a shallow bowl. While the shrimp marinates, prepare the other ingredients.
Peel and deseed the cucumber, then slice into half-moons.
Slice the onion into thin julienne strips.
In a blender, combine the lime juice used for marinating the shrimp with the serrano chili, fresh garlic, and cilantro. Blend until you have a completely liquid sauce, this is going to be our aguachile marinade.
Pour the sauce over the shrimp, then add the onion and cucumber.