Combine water, kosher salt, brown sugar, curing salt, and spices in a large pot. Bring to a boil and dissolve the salts and sugars completely over medium heat, then cool and refrigerate the brine.

Place the beef brisket in a container or plastic bag and pour the cooled brine over it, ensuring it's fully submerged. Seal and refrigerate for 10 days, flipping every few days.

Preheat oven to 300°F (150°C) on the day of cooking.

Chop carrots, celery, and onions and spread in a roasting pan. Add enough water to keep vegetables half submerged.

Place the brined brisket on top of the vegetables. Cover with aluminum foil and bake for 5-6 hours until fork-tender.

Let the brisket rest for 20-30 minutes before slicing. Serve hot against the grain.