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

Heat about 1/2 inch (1 cm) of vegetable oil in a small skillet over moderate heat and fry the tortillas, one at a time, until crisp. Drain on paper towels.

Heat the butter in another skillet over moderate heat and saut?the onion until tender but not brown, about 5 minutes.

Add the tomato and saut?for 3 to 4 minutes.

Add the shrimp and saut?until the shrimp are pink and firm to the touch, about 3 minutes.

Season with salt and pepper and cover the tostadas with the shrimp mixture. Top with the grated Parmesan and the dressed lettuce.

Serves 4 to 6 as an appetizer.