8 ounces watercress (2 qt.)
8 ounces fat-trimmed beef sirloin, about 3/4 inch thick
3 tablespoons lime juice
1 1/2 tablespoons fish sauce or soy sauce
1 tablespoon sugar
1 teaspoon coarse-ground pepper
2 teaspoons vegetable oil
1 onion (6 oz.), peeled and thinly sliced
2 cloves garlic, peeled and minced
Salt

Rinse watercress and drain well. Pick tender sprigs from tough stems and cut into 3- to 4-inch lengths (you should have about 6 cups sprigs); reserve stems for another use or discard. Arrange watercress sprigs in a shallow bowl or on a rimmed serving plate. Rinse beef and pat dry; trim and discard excess fat. Cut into 1/8-inch-thick slices about 3 inches long. In a small bowl, mix lime juice, fish sauce, sugar, and pepper. Set a 10- to 12-inch frying pan over high heat. When hot, add 1 teaspoon oil and tilt pan to coat bottom. Add onion and stir until lightly browned, about 1 minute. Pour onto watercress. Return pan to high heat. Add remaining 1 teaspoon oil and the beef and garlic; stir often until beef is lightly browned, about 2 minutes. Add lime- juice mixture and stir until it boils. Pour from pan over onions and watercress. At the table, mix gently before serving. Add salt to taste.