Preheat oven to 350℉.
Under cold water, thoroughly rinse the brisket to remove any excess brine. Pat dry with a paper towel and place into a large deep roasting pan with the fattiest side facing upward. Set aside.
In a medium bowl or measuring cup, mix 1/2 cup of the Guinness with the black peppercorns, coriander seeds, cloves, celery seeds/flakes, red pepper flakes, ground mustard, nutmeg, and ground ginger. Pour the mixture around the brisket in the roasting pan. You want the bottom inch of the brisket covered. If needed, add in the remaining 1/4 cup of Guinness.
Arrange the bay leaves, garlic, and onion around the corned beef.
Cover with foil and cook for 3 ½ - 4 hours or until the meat is tender. Remove the meat from the pan, leaving in any juices, and set aside to cool.
Increase the oven to 400*.
Add in the baby carrots, chopped cabbage, red potatoes, and 1 cup water into baking pan.
Cover back up with the foil and bake for 30-40 minutes more or until the vegetables are tender.
Remove from oven and let cool for 10-15 minutes.
With a carving knife, cut the meat against the grain and serve with the vegetables.
