In Mexico, "Tacos de lengua" (cow tongue tacos) are popular, and I've come to love them! I want to learn to make them in the U.S. What is the proper way to cook a cow tongue? I've seen the taco stands remove the skin from the already-cooked tongue before slicing it to serve on tacos. But what did they do before then?
Standard prep for beef tongue seems to be: scrub it well and rinse, then soak it in cold water for 2-3 hours. Heat water to a low simmer (about 200F) and cook slowly for at least an hour per pound. Preferably cook as long as you can (5-6 hours) to get a more tender result. When meat is tender and cooked, remove meat and save the broth. Cool the tongue and peel off the outer skin. The broth may be reduced and added to vegetables and meat to create a richer, meatier taco.
I also found a tongue taco (how is that NOT slang for some sexual act?) recipe.
Personally, I'm also a big fan of cold boiled beef tongue with horseradish sauce. You can go from the cooked tongue straight to that just by slicing it up. Now I really want to make tongue tacos.
I've only cooked cow tongue once, so don't pay too much attention to what follows. The tongue should be blanched to remove impurities (slime). Just put it in cold water and bring that to a simmer (as what Bob said, let it soak for 2-3 hours).
Then, change the water, start with cold water and simmer the tongue until done. Add the flavor you want. Check for doneness by sticking a needle in the flesh and there is no resistance.
Remove the skin. I've tried to remove the skin after blanching. No luck, it stuck to the flesh. Try it after cooking.
Having grown up on a farm in Nebraska I can speak from experience. There is no need to soak or blanch the tongue first. Thoroughly wash it is all you need to do. In a large stock pot add the seasoning you prefer. I use a large onion sliced, 4 or 5 bay leaves, dried oregano, fresh basil, garlic salt and Fiesta fajitas seasoning. Lay the tongue in the stock pot and cover it completely with water, then simmer for 1 hour per pound. Remove the tongue from the stock pot and let cool long enough that you can handle it without getting burned, then skin it. Do not let it completely cool or it will be much harder to skin.
Let cool overnight in the refrigerator and then slice crosswise in 1/4" slices. I prefer it cold on bread and butter with a sprinkling of salt and pepper; delicious! You can also make a sauce of sautéed chopped onions and garlic mixed with tomato sauce. Throw in several slices of tongue, let simmer for 30 minutes, remove from sauce and put on fresh hot flour tortillas (Lengua de Res tacos, pure heaven)