Place the corned beef, fat side up, into a large pot (I used a 7 quart Dutch oven) filled with water. Add the spice packet that comes with the corned beef to the water. Cover the pot and simmer over low heat until the beef is tender when pierced with a fork, which takes about 3 hours. (If you plan to chill the beef overnight, keep it in the liquid. The next day, bring it to a simmer before continuing with the recipe.)
1 (3.5 - 5 lbs.) corned beef brisket
Preheat the oven to 450 degrees F. Line a baking sheet with foil, place a rack on top, and coat it with nonstick spray. Move the cooked beef to the prepared rack, positioning the fat side up. Use a knife to trim off the soft and easily removable fat.
Combine the remaining ingredients in a saucepan and bring to a boil over high heat until thickened, typically 4-6 minutes. Spread the glaze over the beef, reserving some sauce. Roast for 10 minutes or until the glaze darkens and becomes sticky. Remove from the oven, cover, and allow to rest for 10 minutes before serving.
1/2 cup ketchup, 1/2 cup whiskey, 1/2 cup apple cider vinegar, 1/4 cup light brown sugar, 1/4 cup low-sodium soy sauce, 2 tsps. ground, dry mustard, 1/2 tsp red pepper flakes
Move the beef to a cutting board and slice thinly against the grain.