Slice beef across the grain. Cut into 2 inches length. Marinate meat in lemon, soy sauce, salt and pepper (by smelling the aroma of the marinated beef, you’ll know if it is well seasoned or not; adjust or correct the seasoning according to your style and liking). Refrigerate for 30 minutes. Pan fry onion rings in hot oil. Cook until onions are soft and translucent. Set aside. In the same pan, stir-fry beef until cooked and tender. Transfer to a serving platter. Garnish with onion rings and serve.