Take the brisket out of the refrigerator about 30 minutes before cooking. Pat it completely dry with paper towels and trim the fat cap down to about 1/4 inch thick.
Stir together the kosher salt, black pepper, smoked paprika, garlic powder, and onion powder. Sprinkle over every side of the brisket and press the seasoning into the meat. Let sit for 10 minutes.
Heat 1 tablespoon of olive oil in a large cast iron skillet over medium-high heat. Sear the brisket fat side down for 4 to 5 minutes without moving it, then flip and sear the other side for 4 to 5 minutes until deeply browned. Transfer to a plate.
Add the remaining 1 tablespoon of olive oil to the skillet and reduce heat to medium. Cook the sliced onions for 6 to 8 minutes until soft and lightly golden. Stir in the garlic for 30 seconds, then the tomato paste for 1 minute. Pour in the broth and scrape up the browned bits.
Add the Worcestershire sauce, brown sugar, apple cider vinegar, and liquid smoke (if using) to the skillet. Stir until the brown sugar dissolves, then remove from heat. Spread half the onion mixture in the bottom of the slow cooker and nestle in the bay leaves and thyme.
Set the seared brisket on top of the onions, fat side up. Spoon the remaining onions and all the liquid over the brisket. Cover and cook on LOW for 8 to 10 hours without lifting the lid.
Start checking at 8 hours by sliding a fork into the thickest part; it should twist easily with little resistance. An instant-read thermometer should read between 195 and 205 degrees F. If still firm, cook another 30 to 60 minutes.
Lift the brisket onto a cutting board and tent loosely with foil for 15 minutes. Remove bay leaves and thyme stems from the slow cooker. Pour the cooking liquid and onions into a saucepan, skim off excess fat, and bring to a simmer. Whisk in the cornstarch slurry and cook 2 to 3 minutes until thickened.
Find the direction the muscle fibers run and slice across them into 1/4 inch thick slices. Arrange on a platter, spoon the warm onion gravy over the top, sprinkle with fresh chopped parsley, and serve right away.