Cut meat lengthwise into 2 or 3 strips; cut strips diagonally into 1/8-inch-thick slices. PLace in a medium-size bowl. Add 2 tbsp soy sauce, 1 tbsp oil, 1/2 tsp sugar and garlic. Mix well. Cover and marinate in refrigerator at least 2 hours. Mix again and bring to room temperature before cooking. Cut onion in half lengthwise. Cut each half into quarters, making 8 wedges. Separate onion layers; set aside. Soak chillies in warm water about 30 minutes or until softened. Discard seeds and stems and mince chillies. Combine chillies, remaining 2 tbsp soy sauce and beef stock in a small bowl. Heat wok over high heat. Add 4 tbsp oil and heat. Add onion; stir-fry 1 minute. Remove onion with a slotted spoon, draining oil back into pan; place onion in a small bowl. Add steak to wok; stir-fry 3 minutes. Remove steak and any liquid in wok; add to onion.

Heat remaining 1 tbsp oil in wok. Add remaining 1/2 tsp sugar. As soon as sugar melts, stir in chili mixture.

Return steak and onion to wok; stir-fry 1 minute. Remove from heat.

Arrange 3 lettuce leaves on each of 4 lightly heated large individual plates or platters. Place 1/4 of beef in center of each serving.

Arrange 3 tomato slices around beef. Arrange 4 cilantro sprigs on each serving.

From the collection of Karen Deck