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.