Preheat the oven to 325 degrees F. Place the halved garlic heads in a single layer on a sheet of foil. Drizzle with the olive oil and season with salt. Fold the foil into a packet; roast until tender, about 45 minutes.
Meanwhile, cook the brisket: Pulse the 6 garlic cloves in a food processor to make a paste. Season both sides of the brisket with salt, pepper, the paprika and chili powder. Heat 2 tablespoons canola oil in a large Dutch oven over medium-high heat. Working in batches if necessary, add the meat to the pot in a single layer and brown on both sides, about 5 minutes total. Remove the meat from the pot and spread with the garlic paste.
Drain the oil and wipe out the Dutch oven. Add the remaining 1 tablespoon canola oil to the pot and warm over medium heat. Add the onions and cook 5 minutes. Season with salt and stir in the brown sugar; cook 5 minutes. Stir in the vinegar and cook 2 minutes. Return the brisket to the pot, add the wine and simmer, 25 minutes. Add the stock and bay leaves and return to a simmer. Cover, transfer to the oven and cook until the meat is tender, 2 hours to 2 hours, 30 minutes.
Remove the brisket from the pot and let rest 15 minutes. Meanwhile, discard the bay leaves and put the pot with the braising liquid over medium heat; simmer until thickened, about 20 minutes. Squeeze the roasted garlic out of its skin and whisk into the liquid. Taste and season with salt and pepper, if necessary. Thinly slice the brisket against the grain and toss with some of the roasted garlic sauce. Warm the tortillas; fill with the brisket and top with slaw.
Red Cabbage Slaw:
Combine 1 small head shaved red cabbage, 4 sliced scallions, 3 tablespoons red wine vinegar, a splash of olive oil and a dab of Sriracha in a bowl; season with salt. Set aside to allow the vinegar to work its magic.