Shrimp and Avocado Ceviche

INGREDIENTS

• 4 limes
• 2 pounds medium shrimp
• 1/4 cup (about 3 limes total) freshly squeezed lime juice
• 6 scallions, trimmed and finely chopped
• 2 to 4 Serrano chiles, finely chopped
• 1/4 cup cider vinegar
• 2 teaspoons finely chopped fresh thyme
• 1 teaspoon dried oregano, preferably Mexican
• 2 large ripe avocados, peeled, pitted, and diced
• 1/4 cup finely chopped fresh cilantro
• 2 teaspoons salt
• Tortilla chips, for serving