Preheat oven to 325°F (163°C) if using oven method.
Rinse corned beef and pat dry. Rub Dijon mustard all over the beef.
Mix garlic, brown sugar, black pepper, and fennel seeds; sprinkle evenly on the corned beef.
Place corned beef in a large dutch oven or slow cooker.
Pour apple juice and apple cider vinegar around the beef.
If using oven, cover and cook for 3 hours 30 minutes until tender.
If using slow cooker, cook on low for 8 hours or until tender.
For extra crispiness (oven method), uncover and broil the corned beef for 5-10 minutes until crispy on top.
Let the meat rest 10 minutes before slicing and serving.