Rub the bottom and sides of the slow cooker stoneware with 1 tablespoon (15 ml / 0.5 fl oz) of the olive oil.
Arrange the sliced onions in an even layer on the bottom.
Heat the remaining 2 tablespoons (30 ml / 1 fl oz) of olive oil in a large skillet over medium-high heat.
Carefully place the brisket in the hot skillet and brown well on all sides, about 4-5 minutes per side. This step is crucial for developing deep flavor.
Transfer the seared brisket to the slow cooker, placing it on top of the onions.
Season the meat all over with the salt, pepper, and dried garlic.
Top the brisket with the sliced celery, bay leaf, thyme, and the entire can of undrained diced tomatoes.
Pour the red wine and beef broth around the edges of the meat, not directly over the top, to avoid washing off the seasonings.
Cover and cook on Low for 6 to 8 hours, or until the brisket is fork-tender.
Carefully remove the meat from the slow cooker and place it on a serving platter or cutting board.
Remove and discard the bay leaf from the pot.
Use a slotted spoon to lift the onions and vegetables from the slow cooker and arrange them around the brisket. Spoon some of the rich drippings over everything.
Garnish with fresh parsley.
Cover the brisket loosely with foil and let it rest for 10 minutes before carving thinly against the grain to serve.
Don't skip the sear! Browning the brisket in a skillet before slow cooking creates a deep, caramelized flavor through the Maillard reaction that you can't achieve from the slow cooker alone.
Place the brisket fat-side up in the slow cooker. As the fat cap renders during the long cooking time, it will self-baste the meat, keeping it exceptionally moist and flavorful.
Resting the meat is non-negotiable.
Let the cooked brisket rest for at least 10 minutes before slicing. This allows the juices to redistribute throughout the meat, ensuring every slice is tender and juicy instead of dry.
