Using a small, sharp knife, butterfly the shrimp open and slice into two pieces.
Rinse the shrimp under cold running water and pat dry with paper towels and transfer
back to shallow glass dish.
To the shrimp, add ½ of the lime juice, making sure the shrimp is covered.
Lightly season with salt to taste. Cover and chill while you prep the rest of ingredients.
Once the shrimp have cooked in the lime juice strain the liquid and reserve.
In the blender, add the serrano peppers, cilantro, oil, reserved lime juice and salt to
taste.
Blend on high until desired consistency. Taste for salt.
Pour this mixture over the shrimp making sure it's mixed in evenly. Season with fresh
cracked pepper.
Fold in the onions, cover and chill for just 15 more minutes.