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.
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.
