Season the beef brisket with smoked paprika, onion powder, garlic powder, salt, and pepper, rubbing the spices into the meat.
In a slow cooker, combine the sliced onions, bay leaves, thyme sprigs, beef stock, and Worcestershire sauce. Stir well to combine.
Place the seasoned beef brisket into the slow cooker, making sure it is submerged in the liquid mixture. Add a little extra salt to taste, if desired.
Cover the slow cooker, and cook on low heat setting for approximately 8-10 hours, or until the beef brisket is tender and easily pulls apart. It should reach an internal temperature of 195 degrees Fahrenheit.
Once the brisket is cooked, carefully remove it from the slow cooker, and transfer it to a cutting board.
Allow the brisket to rest for a few minutes before slicing it against the grain into thin or thick slices, according to preference.
Serve the tender slow-cooked beef brisket with the flavorful onions and a ladle of the cooking liquid.
Store in an airtight container in the fridge for up to four days.