Place water, salt, molasses, garlic and pickling spice into pot on low heat. Stir until salt and molasses completely dissolve. Remove from heat and add 2-4 cups of ice to cool liquid. Should be chilled. Stir in beet juice.
Add brisket. Meat should be totally submerged. Place saucer or plate to hold meat down if necessary. Cover and place in fridge for 5 to 10 days, ideally 7 days. Check daily to ensure beef is staying submerged and to stir brine.
When ready to cook, remove beef and discard liquid. Trim off any excess fat.
Slow Cooker Method
Place the brisket into a large slow cooker, add the onion, carrot and celery and cover with water by 1-inch (about 2 quarts). Cover and cook on low for 8 hours.
Stove Top Method
Place brisket into large pot, cut into pieces if needed. add the onion, carrot and celery and cover with water by 1-inch (about 2 quarts). Bring to a boil, then reduce to a simmer. Partially cover and cook 3 to 4 hours until fork tender.
Leftover corned beef can be stored for up to 5 days covered in the fridge.
To freeze, place corned beef in its juices in a freezer bag. Store for up to 3 months.