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.