Heat a large skillet over medium high heat and sauté onions in vegetable oil, stirring occasionally, until caramelized and most of liquid has evaporated, about 15 minutes. Add halved garlic cloves and saute for 3 minutes more. Stir in spices and seasoning (paprika, salt, garlic and onion powders, black pepper, cayenne, oregano and thyme) and cook for 2 minutes. Set aside. In a large bowl, stir together the beef stock, ketchup, chili sauce and brown sugar. If baking in oven: Preheat oven to 350 degrees. Place brisket in a baking dish or casserole, spread onion mixture over the top, then pour sauce mixture over the entire dish. Cover tightly with foil bake until very tender but not falling apart, about 3 to 4 hours. If making in a slow cooker: Place brisket in a slow cooker, spread onion mixture over the top, then pour sauce mixture over the entire dish. Cover with the lid and cook it on LOW for 10 hours. For both methods, rest the dish: When the brisket is cooked but still hot, use a spoon to scrape off any large fat deposits adhered to the top and bottom of the brisket. If you’re using a slow cooker, transfer the brisket and all of its sauce to a baking dish. If you’ve baked it in the oven, you can continue in that same dish. Chill entire dish in the fridge for several hours and up to one day. An hour before you’re ready to serve it: Preheat your oven to 300°F, and remove the dish from the fridge. Remove all of the fat that has solidified with a slotted spoon for a less oily finish. Carefully remove the meat from its sauce and place on a large cutting board. Cut the brisket into 1/2-inch slices. Carefully place the sliced meat back into the sauce and spoon the sauce over the meat. Replace the lid or cover the dish tightly with foil and reheat in the oven until it is bubbling at the edges — this usually takes up to 30 minutes.