For the stock: Remove the fillets from the fish (or have your fishmonger do it), wrap them, and set them aside in the refrigerator. Rinse the carcass well with water, chop it into 3 pieces, and place it in a stock pot. Add the onion, garlic, serrano, thyme, bay leaves, and peppercorns. Add cold water to cover the ingredients by 1-inch and put the pot over medium-high heat. Bring it to a boil, skimming off any scum that rises to the surface. Reduce the heat and simmer for 20 minutes. Strain and discard the solids, season it with salt. Makes about 6 to 7 cups stock.
For the tortillas: Heat about 1/2-inch vegetable oil in a deep skillet over medium-high heat. Cut the tortillas into 1/2-inch wide strips. Fry the tortilla strips in batches and drain them on paper towels; season with salt and pepper while they are still hot.
For the fish: Drizzle the fish filets with olive oil and lime juice and season with some salt. Cover the fish and put it back in the refrigerator to marinate for at least 15 minutes but no longer than 1 hour.
For the soup: Heat the stock to simmering. Slice the marinated fish into thin strips. Place 3 pieces of fish into the bottom of a large soup bowl. Top with some onion, tomato, avocado, and cilantro leaves. Ladle in the hot stock and squeeze in a lime wedge. Top with tortilla strips and serve.