Spray a 3 1/2 to 4 1/2-quart slow cooker insert with cooking spray. Pour the water in the cooker. In a small bowl, mix the chili powder, brown sugar, salt, oregano, paprika, cumin and pepper flakes. Trim all the visible fat from the brisket. Cut the brisket across the grain in 2-inch chunks. Rub the seasoning mix on all sides of the beef. Place the brisket pieces in the slow cooker. Cover and cook on the Low heat setting for 6 to 8 hours or until the meat can be easily shredded. 30 minutes before serving, mix the cornstarch with 2 tablespoons of water. Remove the meat from slow cooker onto a cutting board or large pan. Shred the meat with 2 forks. Stir the cornstarch mixture into the liquid in the slow cooker. Stir back in the shredded meat. Increase the heat setting to High. Cover and cook for 30 minutes or until the sauce is thickened. Meanwhile, heat the oven to Broil. Place the chiles on the broiler rack. Broil 2 to 3 inches from the heat source for 15 minutes or until the chile skin is charred and blistered, turning every 5 minutes to char all sides. Remove the chiles from the oven and seal in a brown paper bag. Let stand 5 minutes. In a 12-inch nonstick skillet, heat the butter and oil over medium heat. Add the onions. Cook for 15 to 20 minutes or until golden brown and tender, stirring frequently. Peel, seed and slice the roasted chiles. Heat the tortillas as directed on the package. For each taco, spoon about 1/4 cup shredded meat on each tortilla. Top each with the onions, chiles and cheese. Garnish with the sour cream.