Add all the ingredients, except for the tongue, to a large stockpot and bring to the boil. Stir until the salt is completely dissolved, then kill the heat and let the brine come down to room temperature. If at all possible, place it in the fridge and let it cool overnight. To speed up the process, you could also add only half of the water to the stockpot and put the other half in the freezer, then add the cold water to the brine once it has boiled and the salt has completely dissolved.
Once the brine has cooled down, place your tongue into a non-reactive container and pour the brine right over it until it’s completely covered. Now you need to make sure that your meat is completely submerged and that it will remain submerged for the entire duration of the curing process. If it wants to float to the top, weigh it down with a plate or any other similar clean and non-reactive object that fits snugly inside your container.
Place your meat in the fridge and leave it to cure for 12 to 14 days. Check daily to make sure that your meat is still submerged.
BRAISING AND SERVING THE MEAT
After the meat is done curing, rinse it under cold running water and place it in a Dutch oven; add onions, garlic, celery and carrots and cover with cold water. Discard brine.
Cover your beef tongue place it in a 250F oven for about 6 hours or until the meat is super tender and pulls apart when you tug at it with a fork.
Remove the cooked meat from the oven and let it rest for about 10-15 minutes and then peel the skin off. If your meat is cooked all the way, the skin should come right off. If it offers resistance, put it back in the oven and give it a bit more time in there.
Once the tongue has been completely peeled, transfer it to a cutting board, slice it across the grain and serve.