In wok or large frying pan, saute meat in oil over medium high heat. Add shrimp, then onion, mushrooms and garlic powder, stirring after each addition. Sprinkle soy sauce to taste. If desired, add oysters and bean sprouts. Make a well in the center of the mixture and put in about 1 tablespoon of oil. Break up pre-cooked rice and crumble into the oil well, gradually incorporating rice into meat mixture. Stir in scrambled eggs, place on platter and top fried rice with green onions. Garnish the sides of the rice with tomato quarters and lemon slices.