In a medium bowl, stir to combine the chile flakes, chili powder, garlic powder, onion powder, paprika, brown sugar and some salt and pepper. Rub the spice mix on the brisket to coat. Place in a large resealable bag or dish and refrigerate for at least 1 hour or up to overnight.
Preheat the oven to 325 degrees F and position a rack in the center of the oven.
Heat the olive oil in a large Dutch oven set over medium heat. Sear the brisket, fat-side down, until caramelized and browned, 6 to 8 minutes, then flip and cook another 5 minutes.
Pour in the Nancy's Sweet and Tangy BBQ Sauce, Worcestershire and beer. Bring to a simmer, then cover the pot and transfer to the oven. Cook until the brisket is tender and the internal temperature of the thickest part reads 185 degrees F on an instant read thermometer, about 2 hours 30 minutes.
Place the brisket on a cutting board to rest for 20 minutes, and reserve the cooking sauce. Slice the brisket thinly against the grain, then return the meat to the Dutch oven and coat in the sauce. Pile the meat onto the sandwich rolls, spread some mayo on one side of the roll and then drizzle some extra sauce over the meat to serve.
In a medium saucepan, heat the butter over medium heat. Add the onions and cook until soft and translucent, 4 to 5 minutes. Stir in the garlic and cook 1 minute, then whisk in the tomato sauce, ketchup, sugar, vinegar, mustard, cayenne and some salt. Bring to a boil, then reduce to a simmer and cook for 45 minutes.