Rinse tongue and put in a large pot and cover with water, quartered onion, crushed garlic cloves, peppercorns, bay leaves, and a couple pinches of salt. Bring to a boil, then partly cover, reduce to a simmer and cook for around 2 hours until fork tender.
Once cooked, remove from water and allow to cool for a few minutes. Remove outer layer of tongue and cut into thin slices.
Add diced tomatoes, diced red onion and 2 tbsp chopped cilantro into bowl Add salt and pepper to taste. Gently mix.
On a grill pan (or sautee pan) quickly sear slices of beef tongue.
Add sliced tongue, salsa, avocado slices and garnish with a wedge or lime.