Make the brine: Fill a large pot with the water, along with the salt, brown sugar, pink salt, black peppercorns, coriander, mustard seeds, red pepper flakes, juniper, allspice, cloves, bay leaves, onion and garlic. Bring to a boil over high heat until the salt and sugar dissolve, 6 to 8 minutes. Remove from the heat and let cool to room temperature.
Add the brisket to the brine and use a plate to weigh down the meat and keep it completely submerged. Cover and refrigerate for 6 days, stirring the brine every 2 days.
Make the corned beef: After 6 days, remove the brisket from the brine and pat dry. Place the brisket in a pot large enough to hold the meat, and cover with water by 1 inch. Add the bay leaves, carrots, onion and celery to the pot. Wrap the coriander and peppercorns in a cheesecloth and tie using kitchen twine, and add to the pot.
Bring the corned beef to a boil over medium-high heat. Reduce the heat to medium low, cover and gently simmer until the beef is tender, 2 hours and 45 minutes to 3 hours. Remove the brisket and allow to cool for 10 minutes before slicing. Cut against the grain into ¼-inch slices and serve with horseradish cream and stout mustard. Make ahead: The brisket can be made the day before. Transfer the brisket to a baking sheet, chill in the fridge before slicing and rewarm with the braising liquid over medium heat.