Rinse the corned beef under cold water to remove brine and pat dry with paper towels.
Crush the spice packet seeds using a mortar and pestle or heavy skillet.
Mix crushed spices with minced garlic, brown sugar, fennel seeds, and black pepper.
Spread Dijon mustard evenly over the fatty side of the brisket.
Press the spice and sugar mixture firmly over the mustard to form a crust.
Choose your cooking method: Oven or Slow Cooker.
For oven method, preheat oven to 325°F and place brisket fat side up in a Dutch oven with apple juice and cider vinegar. Cover tightly and bake about 50 minutes per pound until tender.
For slow cooker method, place brisket fat side up in slow cooker with apple juice and cider vinegar. Cook on LOW for 6-8 hours until tender.
Remove cooked brisket and place on foil-lined baking sheet.
Set oven to broil and place brisket about 6 inches from heat. Broil for 5-6 minutes until crust is deeply caramelized but not burnt.
Rest brisket for 10 minutes before slicing and serving.