1. Make the dipping sauce by stirring together the ingredients with a whisk to combine.
2. Heat oil for sautéing in a medium frying pan. Sauté onion over medium heat until golden, approximately 15 minutes. Add sliced tongue and sauté for five more minutes. Add mustard, sugar, and salt; mix to combine. Remove from heat.
3. Fill spring roll wrappers with the meat mixture and sprinkle with scallions. Roll them spring roll style. You can either fry the spring rolls in hot oil or bake at 450 degrees Fahrenheit (220 degrees Celsius) for approximately 20 minutes.
4. Reheat uncovered. Serve with dipping sauce.