- In a stockpot over medium heat, combine beef tongue and enough water to cover. Bring to a boil for about 10 to 15 minutes, skimming scum that may float on top. Drain tongue and rinse well. Discard liquid and wash pot. 
- Return tongue to pot and add enough water to cover. Add onions, garlic, bay leaves, peppercorns, and about 2 tablespoons of salt. Over medium heat, bring to a boil.  
- Lower heat, cover and simmer for about 3 to 4 hours or until tongue is cooked through and fork-tender. Add more water as needed during cooking to cover tongue. 
- Drain well, reserving 2 cups of the liquid. Strain and discard aromatics. 
-  Allow tongue to slightly cool and refrigerate for about 1 to 2 hours or until completely cooled (this will make slicing the tongue easier). Peel the skin and cut meat diagonally into ½-inch slices. 
- In a bowl, combine cream of mushroom soup and reserved broth. Stir until well-blended and smooth. 
- In a wide pan over medium heat, melt butter. Add mushrooms and cook, stirring regularly, until lightly browned. Remove from pan and set aside. 
- Add onions and cook until softened.  
- Add beef tongue and cook, stirring regularly, for about 2 to 3 minutes or until lightly browned. 
- Add diluted mushroom soup and stir until completely dissolved and sauce is smooth. 
- Season with salt and pepper to taste. Add mushrooms.  
- Continue to cook for about 8 to 10 minutes or until sauce is reduced and thickened to desired consistency. Serve hot.