Preheat the oven to 300℉.
In a small mixing bowl, combine the salt, pepper, ground mustard, cumin, smoked paprika, chili powder, and cayenne.
Using paper towels, pat dry brisket. Cut the brisket in half to create two large pieces. Using your hands, evenly coat both sides of brisket with the spice mixture.
Heat the oil in a heavy-bottomed and oven-safe pot or Dutch oven over medium-high heat. Working in batches as needed, add the brisket and sear on both sides until golden-brown crust forms, 4 to 5 minutes per side.
Meanwhile, in a medium-sized bowl, combine the bourbon, ketchup, coconut aminos, apple cider vinegar, garlic, sugar, Dijon mustard, and beef broth. Set aside.
Transfer the seared brisket to a separate plate and turn off the heat. Pour THE sauce into the pot and stir, scraping up any browned bits from the bottom of the pot. Nestle the brisket back into the sauce.
Add the sliced onions and cover with a lid. Transfer to the oven and bake until brisket is cooked through and fork-tender, 3 to 3½ hours.
Slice and serve the brisket on a large platter. Drizzle a little of the sauce from the pot over the brisket and serve the remaining sauce in a bowl on the side.