Place the tongue in a large saucepan and fill with water to cover the tongue by 1 inch. Bring to a boil, skimming occasionally. Stir in the beef or vegetable stock and return to a boil. Reduce heat to a simmer, cover, and cook for 2 hours. Remove the tongue from the stock and place it on a cutting board. Make a small incision at the top of the tongue and peel away the skin, discarding it. Let the tongue cool completely, preferably overnight. Preheat the oven to 300F. Grease a large gratin dish with 2 tbsp of the butter and sprinkle with 2 tbsp of the Parmesan cheese. Cut the tongue into thin slices and place a layer in the gratin dish. Spoon over 1/3 of the tomato sauce and 1/3 of the remaining Parmesan cheese. Repeat layers, beginning with tongue, until all ingredients are used up. Melt the remaining butter and pour it over the gratin. Place gratin in oven and bake for 25-30 minutes, until golden-brown and bubbling.
