Combine all the ingredients in a large pot and bring to a vigorous boil over high heat. Remove the pot from the heat and let the brine cool to room temperature.
Fully immerse beef tongue in the solution in a large nonreactive container. (A gallon-size heavy-duty sealable plastic bag works nicely, but make sure you set it in a large bowl in case it leaks.) Place the container in the fridge or a large cooler and let it mellow out for 24 hours.
:::pickled mustard seeds:::
Place the mustard seeds in a small pot, cover with cold water, and bring the liquid to a boil. Strain the mustard seeds, discarding the water, and repeat this process 3 more times.
In the same pot, combine 1 cup of cold water with the vinegar, sugar, salt, and the blanched mustard seeds. Bring to a boil, and then remove from the heat. Pour the contents into a small sealable container and refrigerate it overnight.
When you’re ready to use the pickled mustard seeds, scoop them from the brine, straining off any excess liquid.
:::tongue:::
Set a slow cooker on the high setting, add the olive oil, and heat it until it begins to shimmer. Then add the onion and garlic and cook for 10 minutes, until softened. Add the stock and the mustard, and cook for 5 minutes.
Add the beef tongue, bay leaves, salt, pepper, and caraway to the slow cooker. Cover with the lid, reduce the setting to low, and cook for 8 hours.
Once it is tender, remove the tongue from the slow cooker and let it cool until it is comfortable to handle. Carefully remove the tough exterior membrane by simply peeling it off with your hands. Return the tongue to the braising mixture and leave it there until you are ready to eat. Before serving, thinly slice the beef tongue against the grain. Serve it up with some Pickled Mustard Seeds.