1 pound firm, mild white fish fillets, such as flounder, snapper, or sea bass, cut into 1⁄2-inch dice
2 cups freshly squeezed lime juice (from 12 to 14 limes)
6 Tbsp. olive oil
2 garlic cloves, peeled
2 dried chiles de árbol
1 pound large shrimp, peeled and deveined
2 tomatoes, finely chopped
1 to 2 jalapeños, finely chopped
2 pickled jalapeños, finely chopped, or to taste
2⁄3 cup finely chopped white onion
1⁄2 cup chopped fresh cilantro leaves and upper stems
1⁄3 cup pimiento-stuffed green olives
1⁄4 cup chopped drained capers
1 1⁄2 cups ketchup
1 1⁄2 tsp. dried oregano
1 pound jumbo lump crabmeat, picked over for shells and cartilage
1⁄4 tsp. freshly ground black pepper, or to taste
1 to 2 ripe avocados, halved, pitted, and sliced or diced
Corn tostadas, tortilla chips, or saltine crackers for serving
Place the diced fish in a bowl, add the lime juice, and gently mix. Cover with plastic wrap and marinate for at least an hour, and for up to 12 hours, in the refrigerator.
Heat the olive oil in a small skillet over medium-low heat. Add the garlic cloves and chile de árbol and cook until the garlic is golden, 10 to 15 minutes, stirring occasionally; take care not to burn the garlic and chiles. Turn off the heat and allow the oil to cool, then remove and discard the garlic cloves and chiles.
Bring a medium saucepan of water to a boil, salt generously, and add the shrimp. Cook for 1 minute, or just until pink. Immediately drain the shrimp and transfer to a bowl. Let cool. In a large bowl, combine the tomatoes, fresh and pickled jalapeños, onion, cilantro, olives, capers, ketchup, and oregano and mix well. Stir in the seasoned olive oil.
Drain the fish, reserving the marinade, and add the fish, shrimp, and crabmeat to the bowl, along with 11⁄2 cups of the marinade from the fish. Gently mix together, then season with 2 teaspoons salt and the black pepper. Taste and adjust the salt and pepper.
Serve in small bowls or martini or other stemmed glasses, topped with the avocado and with tostadas, chips, or crackers on the side.