In food processor or blender, combine garlic, scallions, salt, sugar and pepper. Process to a paste. Combine the paste with the meat and allow to rest for 2 hours in refrigerator. Add rice powder. Form into meatballs and skewer on sticks. Grill over hot coals or bake in a 300 degree oven for 15 minutes. When serving, give each person a bowl of water, rice papers and some meatballs. They should wet the rice paper which will become pliable in a minute. Stuff the rice paper with meatballs and layer in lettuce, tomato, cilantro and mint. Roll together and serve with fish sauce if desired.