America's Test Kitchen. Makes 2 tongues. You can find salt peter online or at butcher shops.
PICKLING SPICE
1 tablespoon whole black peppercorns
1 tablespoon mustard seeds
1 tablespoon coriander seeds
1 tablespoon red pepper flakes
1 tablespoon allspice
1 cinnamon stick, crushed
8 dried bay leaves, crumbled
1 tablespoon cloves
2 teaspoons ground ginger
TONGUE AND BRINE
4 quarts water
1 cup brown sugar
2 cups kosher salt
1 ounce salt peter (optional)
¼ cup pickling spice
4 cloves garlic, crushed
2 (2½ pounds) beef tongues
Method
For pickling spice: Combine peppercorns, mustard seeds, and coriander seeds in small skillet over medium heat until fragrant, 1 to 2 minutes.
Transfer to mortar and pestle or spice grinder and crush until coarsely cracked.
Transfer to medium bowl. Add remaining ingredients and toss to combine.
For tongue and brine: Combine water and sugar in large pot and bring to boil, stirring occasionally.
Once boiling, add salts and stir until dissolved. Add 2 tablespoons pickling spice and garlic.
Remove from heat and cool to room temperature; refrigerate brine until chilled.
Place tongues in brine and weigh down with plate to keep submerged.
Refrigerate 5 to 7 days, turning tongues every couple of days.
Adjust oven rack to lower middle-position and heat oven to 300 degrees.
Remove tongues from brine and rinse thoroughly under cool running water.
Place tongues in Dutch oven, adding enough water to cover them completely.
Add remaining 2 tablespoons pickling spice and bring to a simmer.
Transfer Dutch oven to preheated oven and cook until fork-tender, about 3 hours.
Remove tongues from cooking liquid and let sit ten minutes. Peel off and discard outer skin of tongues.
Refrigerate until completely cooled. (Tongues can be wrapped tightly and refrigerated up to four days and frozen up to 2 months.)