Cut the meat into 1-inch cubes. Place in a large mixing bowl. In a small mixing bowl, combine the fish sauce, water, cornstarch, sugar, baking powder and sesame oil. Season with black pepper. Whisk well and add the beef. Toss well. Cover and refrigerate overnight. Remove from the refrigerator and place in the bowl of a food processor, fitted with a metal blade. Process until smooth. To test, bring a small pot of water to a boil, add a teaspoon of the mixture to the water and cook for about 1 minute. Remove from the water, taste and adjust seasoning. Roll the rest of the mixture into individual balls, about 1 tablespoon for each ball. In a saucepan, bring the beef stock to a boil. Add the meat balls, stirring occasionally. When the meat balls start to float, cook for an additional 5 minutes. Stir in the green onions, Nuoc Cham and black pepper. Ladle into individual bowls and serve.
Stem the chile. Split the chile in half, remove and discard the seeds and membrane. Finely chop the chile. Combine the chopped chile, garlic and sugar in a mortar. Pound the mixture into a paste. Remove the zest and pith from the lime, and discard. Add the lime to the chile mixture. Mash well. Add the fish sauce and the water. Mix well. Serve as a seasoning agent instead of salt and pepper.