Place the tongue together with the rest of the ingredients in a pot / casserole. Cover with lid and bring to a boil. Once boiling, reduce the heat and simmer for 2 ½ hours.
Remove from heat and set aside, then allow to cool slightly. Once it has cooled enough for you to handle with ease, remove the skin, then let it cool completely before slicing.
For the mustard sauce:
Combine the egg yolks, sugar, mustard powder and vinegar in a mixing bowl and whisk with a hand mixer. Now place in a small saucepan and add the stock (I use the stock the tongue was cooked in). Bring to a simmer while whisking, then remove from heat and let cool slightly.
Spread the mustard on a serving platter, then top with the sliced tongue. Garnish with fresh herbs and serve with the remainder of the mustard sauce.