Pat dry brisket with a paper towel, and place on a plate. Generously salt and pepper all sides of the brisket. Set aside.
Preheat your slow cooker to 500°F on brown/sauté setting. Alternatively, if you do not have this setting, you can sear brisket in the skillet over high heat.
Dice half of the onion. In a large skillet over medium heat, add the butter and diced onions. Sauté until translucent and fragrant, about 5 to 10 minutes.
Sear all sides of the brisket in the slow cooker, each side will take about 1 to 2 minutes.
Switch slow cooker to the low setting. Add the onions to the slow cooker, half under the brisket and the other half on top of the brisket. Make sure the brisket is fat side up.
Add the bone broth and Worcestershire sauce to the slow cooker.
Slow cook for 6 hours.
Remove ½ cup beef broth from the slow cooker and add to a large skillet with the red wine. Heat on high, and once simmering slowly add in the flour while whisking to combine completely.
Add tomatoes, garlic, and tomato paste to the skillet and whisk to combine completely. Allow to simmer on low, stirring occasionally, for about 10 minutes or until it has thickened slightly.
While the wine mixture is simmering, rough chop the remaining half of onion, celery, and carrots and set aside with the potatoes.
Optional: If want a leaner stew, you can remove some of the fat from the brisket at this time.
Pour the wine mixture into the slow cooker and add in the potatoes, onions, carrots, celery, and bay leaves.
Cook on low for an additional 2 hours, or until the brisket is tender to your liking. Before serving, shred beef into small pieces and stir stew.
Before serving, shred beef into small pieces and stir stew.