canola or vegetable oil
large sweet onions, thinly sliced
(6-ounce) can tomato paste
cloves garlic, minced
kosher salt, plus more for the meat
freshly ground black pepper, plus more for the meat
low-sodium beef broth
apple cider vinegar
packed light brown sugar
(3-pound) beef brisket (preferably flat cut)
Heat the oil in a large frying pan over medium-high heat until shimmering. Add the onions and cook, stirring occasionally, until softened and beginning to brown and char in some spots, about 8 minutes.
Add the tomato paste, garlic, paprika, oregano, onion powder, garlic powder, salt, smoked paprika, cumin, cinnamon, and tomato paste, and cook until fragrant, about 2 minutes more. Deglaze the pan with 1 cup of the broth, using a wooden spoon to scrape the browned bits from the bottom of the pan. Add the vinegar and sugar and stir to combine and dissolve the sugar. Bring to a boil. Reduce the heat to low and simmer for 10 minutes. Transfer to a 6-quart or larger slow cooker.
Pat the brisket dry with paper towels. Generously season both sides with salt and pepper. Place the brisket in the slow cooker fat-side down.
Cover and cook on the LOW setting for 6 to 8 hours. The brisket is done when it easily shreds when pulled, but it shouldn’t be falling apart.
Transfer the brisket to a clean cutting board. If desired, use a spoon to scrape the fat cap off. Now you can slice and serve the brisket immediately with the sauce.