In an 8-quart or larger stockpot, combine 12 cups water, kosher salt, garlic, brown sugar, pink salt, and 3 tablespoons of pickling spice. Bring the liquid to a boil, stirring until the sugar and salt dissolve. Cover the pot with a lid to help the liquid boil faster, checking on it every 5 minutes. Once boiling, remove from heat and let cool for about 30 minutes.
Prepare a large bucket, tub, or cooler that can hold the brisket and brine snugly. Line it with a brining bag, then add 4 cups ice cubes. Pour the semi-cooled brining liquid on top. If the liquid is still warm, the ice will melt and cool it down further. Allow this mixture to rest until it reaches room temperature, about 1 hour.
Carefully place the brisket into the fully cooled liquid, ensuring that it's submerged. If the brisket floats, weigh it down with a heavy pot or a bag of ice. I like to top mine with a stack of dinner plates that is about the same circumference as the bucket to keep it submerged.
Let the brisket brine for at least 5 days, or up to 7 days, making sure it stays below 40 degrees but above 32 degrees to avoid freezing. You can store it in your refrigerator, garage, or outside if the weather permits.
When you're ready to cook, remove the brisket from the brine and discard the brine. Rinse the brisket thoroughly under cold water, then pat it dry with paper towels. Trim off any excess fat using a paring knife—be careful not to cut away any meat.
Place the brined brisket in a large stockpot (at least 6 quarts). Add water until it's at least an inch above the brisket. Add peeled garlic cloves, 2 bay leaves, and 1 tablespoon pickling spice.
Bring the liquid to a boil over high heat. Once boiling, reduce the heat to maintain a low simmer. Cover the pot and let it simmer for about 3 hours. The brisket should become fork-tender and register between 180 degrees F to 190 degrees F on an instant-read thermometer.
After cooking, turn off the heat and allow the meat to cool for about 10 minutes. Carefully use tongs to remove the corned beef and transfer it to a cutting board.
Remove 2 cups of the cooking liquid from the pot and strain the remaining liquid into a bowl. Set aside the strained liquid and discard the rest carefully.
Let the corned beef sit at room temperature for 20 minutes before slicing. When ready to serve, cut the brisket against the grain into 1/4 inch strips.
