Place beef tongue, water, onion, garlic, and sea salt in a slow cooker. Cook on low for 6 to 8 hours.
Once it’s tender, take the tongue out of the stock to cool off. Peel off the outer layer of skin when you are able to handle the tongue. It’s easier to take the skin off while the tongue is warm. Dice tongue into 1⁄2-inch cubes and set aside.
Strain the stock and set aside. It’s always better to refrigerate the stock so you can skim the fat off and discard. If you have a fat separator then you don’t have to refrigerate.
In a large skillet, heat oil over medium heat. Add onion and sauté until translucent, about 2 minutes. Add flour and stir for 2 minutes. Add diced tomatoes and let come to a boil as you stir.
Add diced tongue, 1⁄2 cup reserved stock, black pepper, and garlic powder. Lower heat and simmer for 5 minutes to thicken. If it’s too dry you can add more stock, however, this recipe should not be runny if using as a filling or topping.
In heavy skillet heat one-quarter inch oil. Fry tortillas, one at a time, in hot oil for 20 to 40 seconds on each side or until crisp and golden. Drain on paper towels. Store bought tostadas can also be used.
Top one tostada with lengua. Drizzle with your favorite homemade salsa to taste.
Garnish each with toppings of your choice.
Place lengua on tostadas and add your favorite toppings.