- With the rack in the lowest position, preheat the oven to 450°F (225°C). Line a baking sheet with parchment paper.
- Place the fish fillets on the prepared sheet. Season with salt and pepper. Spoon the salsa verde onto the fillets. Bake for 8 to 10 minutes or until the fish is cooked through. With a fork, break the fish into large flakes. Set aside.
- Meanwhile, in a small bowl, cover the onion with the ice water. Add 2 tbsp (30 ml) of the lime juice and the salt. Let marinate for 15 minutes. Drain and set aside.
- In a bowl, combine the marinated onion, lettuce, cucumber, cilantro, pepper, olive oil and the remaining lime juice.
- Place the tostadas on plates. Spread with the sour cream and top with the fish. Garnish with the salad.
Tostadas are flat crunchy corn shells about 5 inches (13 cm) in diameter. They can be found in the Mexican products section at most grocery stores. If unavailable, replace them with wheat tortilla shells toasted on both sides in a hot skillet.