First, prepare the shrimp by peeling, deveining, removing the stems, and either butterflying them or slicing completely in half. Place these in a bowl along with ¼ cup lime and a pinch of salt.

Let the acid from the lime cook the shrimp for at least 30 minutes, flipping once or twice, until all sides have turned from gray to white/pink.

Meanwhile, in a blender, add serrano peppers (optional to quickly char them on the stovetop first) (also optional to remove the seeds for a less spicy recipe), garlic, ¼ cup lime juice, a pinch of salt/pepper, and ½ of the cilantro bunch (including stems). Blend until smooth.

After shrimp has fully cooked in lime, add the cucumber and red onion slices to the shrimp and pour over the green sauce. Cover and place in refrigerator for about 30 minute.

When ready to serve, add avocado slices and top with some fresh cilantro leaves. Serve cold atop the tostadas.