1. Combine paprika, onion powder, 1 teaspoon salt, garlic powder, and cayenne in bowl. Using fork, prick brisket all over. Rub spice mixture over brisket, wrap tightly in plastic wrap, and refrigerate for 8 to 24 hours. Unwrap brisket and place in slow cooker.

2. Heat oil in 12-inch skillet over medium-high heat until shimmering. Add onions, sugar, tomato paste, and garlic and cook until onions are softened and lightly browned, 8 to 10 minutes. Stir in flour and cook for 1 minute. Slowly whisk in broth, scraping up any browned bits and smoothing out any lumps; transfer to slow cooker.

3. Stir 2 tablespoons vinegar, thyme sprigs, and bay leaves into slow cooker. Cover and cook until beef is tender, 9 to 11 hours on low or 5 to 7 hours on high.

4. Transfer brisket to cutting board, tent loosely with aluminum foil, and let rest for 20 minutes. Let braising liquid settle for 5 minutes, then remove fat from surface using large spoon. Discard thyme and bay leaves. Stir in remaining teaspoon vinegar and season with salt and pepper to taste.

5. Slice brisket ½-inch thick against grain and arrange on serving platter. Spoon 1 cup sauce over meat and serve with remaining sauce.