Place brisket, fat side up, in the slow cooker. Spread the dijon mustard over the top of the brisket and sprinkle remaining 1/3rd of onion mixture over the top. Set slow cooker on low for 9 hours. When brisket is cooked, carefully remove it from the slow cooker and place it in a large roasting pan.
Place the brisket in a large bowl and cover the top and bottom with 2/3rds of the onion mixture. Place brisket, fat side up, in the slow cooker. Spread the dijon mustard over the top of the brisket and sprinkle remaining 1/3rd of onion mixture over the top. Set slow cooker on low for 9 hours.
Mix the garlic powder, onion powder, paprika, salt and pepper in a small bowl and sprinkle over the brisket, patting it down all over the meat. Heat the avocado oil over medium high heat in a Dutch oven. Sear the brisket on all sides for about 5 minutes each side.
Mix the garlic powder, onion powder, paprika, salt and pepper in a small bowl and sprinkle over the brisket, patting it down all over the meat. Heat the avocado oil over medium high heat in a Dutch oven.