Rinse the tongue and place it in a large stockpot. Add enough water to cover - I use 14 cups of water.
Bring to a boil. This should take 20-30 minutes. When the water starts boiling, skim the foam off from the top with a spoon. Lower the heat to a simmer. Add the bay leaves, peppercorns, and allspice.
Cook partially covered, turning the tongue every hour to ensure even cooking on all sides. Cook 3 hours for a 2-pound tongue and 4 hours for a 3-pound tongue.
Remove the cooked tongue to a cutting board. Allow it to cool until it is easier to handle, and then use your hands to peel off the skin.
Slice the tongue and serve with or without sauce.
To make the sauce:
Heat a large, deep skillet over medium heat. Add the butter and swirl to coat. Add the onions and cook, stirring occasionally, until golden, for about 10 minutes.
Add the mushrooms and cook, stirring, until soft, for about 5-7 minutes.
Add beef broth, salt, and pepper. Bring to a boil over high heat. Lower the heat to a simmer, and cook, uncovered, for about 30 minutes to thicken the sauce. If using, sprinkle the sauce with chopped parsley. To serve, arrange tongue slices on plates and top them with the sauce. Serve immediately.
Video
Notes
The nutrition info assumes a 2-pound tongue.
I don't add salt to the cooking water to ensure the meat remains as tender as possible. If not using the sauce, sprinkle the cooked tongue slices with salt to taste.
The cooking liquid can be used as broth. Salt it to taste.
Let the cooked tongue cool a little before removing the skin. It's too hot to handle when you remove it from the saucepan. But don't let it sit and cool for too long. Remove the skin as soon as you can. If it cools down too much, the skin becomes difficult to remove.
If you wish to thicken the sauce further, mix 1 teaspoon of cornstarch with 2 teaspoons of cold water and stir the mixture into the sauce. Cook for 1-2 more minutes until the sauce thickens.
Keep the leftovers in the fridge, in a sealed container, for 3-4 days. Reheat them gently, covered, in the microwave at 50% power, or enjoy them cold like roast beef.