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.