Season both sides of the brisket generously with salt, pepper, and cayenne. For best results, place brisket over an aluminum foil “rack” set over a plate and refrigerate, uncovered, 8 to 12 hours.
Preheat the oven to 325 degrees F (165 degrees C).
Melt butter in a saucepan over medium heat. Add onion and salt. Cook and stir until translucent, about 5 minutes. Stir in garlic, rosemary, and apple juice. Increase heat to high and boil until liquid is reduced by about half, about 3 minutes.
Transfer half of the apple-onion mixture into a baking dish. Place brisket fat-side up on top; cover with remaining apple-onion mixture. Cover baking dish tightly with foil.
Bake in the preheated oven for 1 hour 30 minutes. Reduce oven temperature to 250 degrees F (121 degrees C) and bake until fork-tender, about 2 hours 15 minutes. Check for doneness after 2 hours.
Transfer brisket to a plate and tent with foil.
Pour the onions and braising liquid into a large measuring cup or bowl. Skim some fat off the top. Puree with a stick blender to create a gravy.
Slice brisket across the grain and serve with the gravy.