Add the shrimp and red onion to a shallow dish. Refrigerate for about 10-15 minutes until shrimp begin to turn pink. Flip over and refrigerate for an additional 10-15 minutes until pink and “cooked” through. This "cooking" step is optional since traditionally aguachile is served immediately with fully raw shrimp. However, we find the shrimp is more flavorful after it cooks and marinates in the lime juice.