Start with the briskets at room temperature. Pat the briskets dry with a paper towel and season all sides with salt and pepper. It's easier to season one of the sides when the other side is down in the skillet.
Cover the bottom of a large skillet with olive oil and bring it to medium to high heat. It's important that the oil is fully heated for the next step.
Add the briskets to the skillet one at a time and cook each on both sides until browned, about 2–3 minutes. Transfer the briskets to the slow cooker fat side down.
Reduce the heat to medium then add the sliced onions and 2 teaspoons of salt to the skillet. Add more oil if necessary. Cook the onions turning them with tongs to fully coat them with the oil until they're soft, about 6–7 minutes. Add the garlic and cook another minute or so. Transfer the onion mixture to the slow cooker.
Add the wine to the skillet and deglaze the skillet with a spoon, scraping up any stuck bits. Transfer the wine to the slow cooker.
Add the beef broth and the Worcestershire sauce to the slow cooker.
Cook on low for 8 – 10 hours.
Transfer the briskets to a cutting board and let them rest for at least 5 minutes. Slice the briskets against the grain or shred them with forks.
Turn the slow cooker up to high and skim off the fat. Put some of the au jus in a separate bowl and thicken with corn starch. Then mix it back into the slow cooker. An immersion blender works well to thicken the gravy in the slow cooker.
Serve the briskets and gravy together over mashed potatoes, noodles, or French rolls. Enjoy!