Heat the oil in a large, nonreactive skillet over medium heat, add the onions, and cook for 10 minutes, or until soft. Add the garlic, paprika, and chili powder and cook for 30 seconds, stirring. Add the remaining ingredients, except for the brisket, and cook until heated through. Remove the skillet from the heat.
Pour half of the sauce into a large slow cooker. Add the brisket and cover with the remaining sauce. Cover and cook on low for 8 hours.
Transfer the brisket to a large casserole and ladle all of the remaining sauce over it. Cool to room temperature. Cover with plastic wrap and refrigerate overnight.
Preheat the oven to 350°F. Transfer the meat to a cutting board. Cut the meat across the grain into 1⁄3-inch-thick slices.
Pour the sauce into a large saucepan and bring to a simmer. Return the sauce to the casserole and add the sliced meat. Bake for 30 minutes, uncovered. Transfer the meat slices to a platter and spoon the sauce over the meat.