Without getting into serious cooking, prep as in larding for Flamande, but just keeping it straight ahead...
For a 3 lb already "pickled tongue:
Simmer 2 hours. Remove from the water, peel and remove the roots. Brown some vegetables in butter in a heavy casserole, pour about a quart of liquid in (could be beer, wine or broth), and add the warm tongue to the casserole. Cover, and into a 300F oven for a two hour braise.
Your choice of vegetable will do a lot to determine the character of the tongue. For instance, you could choose a regular mirepoix or the trinity for a European or Creole style respectively.
Classic technique includes rolling the tongue in cheesecloth, and trussing -- after simmeting and trimming -- so it holds a shape during the braise. I find this helps the tongue to slice better later on, but it's not absolutely necessary.
If you do go Euro, try this: Include a good sized turnip (medium diced) in the mirepoix. After sauteeing the vegetables, add a little tomato paste and brown it slightly to make a pincage. Use about 1/2 beef or brown stock, 1/2 beer or wine for the braizing liquid. Also use a bouquet garni and a couple of bay leaves. After the braise, set the tongue aside on a hot, covered platter. Then strain the cooking liquid through a medium sieve, pressing through as much of the mirepoix as you can. That should be the carrots and the turnips, leaving behind the celery fibre and onion. Return the sauce to the heat and reduce it to about two cups. Thicken the sauce slightly with either a flour or corn starch slurry. If you like, you may add your choice of carrots, pearl onions, pieces of parsnip, bell pepper, etc., which you've already blanched or sauteed to crisp-tender.
In the north of France, the Low Countries, and Germany they like to serve tongue on a bed of cooked greens, spinach for instance.
Of course, if you're using different braising vegetables and liquids, you're free to create your own sauce according to whichever classic or improvised techniques you like. Sweet and sour compliments tongue nicely.
In any case, slice the tongue against the grain and only against the grain. An ideal tongue slice is a very tender 3/8" thick.
When the tonuge has cooled it may be sliced very thin and reheated or served room temperature for sandwiches. Tongue and very hot mustard do well together.