Preheat oven to 300 degrees.
In a bowl, mix salt, pepper, ground mustard, cumin, smoked paprika, chili powder and cayenne to create your rub.
Using paper towels, pat dry brisket. Cut brisket in half, to create two large pieces about 2 lbs each. Using your hands, evenly coat both sides of brisket with your rub mixture.
Heat a heavy-bottomed and oven-safe pot, or dutch oven, over medium-high heat with avocado oil. Add brisket and sear on both sides until golden-brown crust forms, about 4-5 minutes per side. You may need to do this in two batches.
While brisket is searing, in a medium-sized bowl combine the bourbon, ketchup, coconut aminos, apple cider vinegar, garlic, sugar, dijon mustard and beef broth. Whisk until well-combined and set aside until beef is done searing.
Set browned brisket aside and turn off the heat. Pour sauce into a dutch oven, breaking up any brown bits at bottom of the pan. Nestle brisket into the sauce.
Top with sliced onion and cover with a lid. Transfer into the pre-heated oven and bake until brisket is cooked through, about 3-3.5 hours. The meat should be fork-tender when ready.
Slice and serve the brisket on a large platter. Drizzle a little of the sauce from the pot over the brisket, and serve the sauce in a bowl on the side for guests to add more as desired!