Good-Cooking - Shrimp Tostadas (Tostadas de Camaron)

Ingredients

• Vegetable oil for frying
• 4 to 6 (6-inch, 15 cm) corn tortillas
• 1 tablespoon (15 ml) butter
• 1 medium onion, finely chopped
• 1 large tomato, seeded and finely chopped
• 1/2 pound (225 g) peeled shrimp, chopped
• Salt and freshly ground pepper to taste
• 1/4 cup (60 ml) freshly grated Parmesan cheese
• 1 to 2 cups (250 - 500 ml) finely chopped lettuce,
• lightly dressed with oil and vinegar