Place beef in a slow cooker and top with Simmer Sauce. Cover slow cooker and cook beef on LOW heat for 8 hours.
20 minutes before serving, transfer beef to a cutting board and allow to rest 5 minutes, then using two forks, shred into bite size pieces.
Drain and reserve liquid from slow cooker and set aside.
Return shredded beef to slow cooker then pour in Finishing Sauce and toss to evenly coat, adding in some of the reserved liquid to thin sauce to reach desired consistency. Increase slow cooker heat to HIGH, cover slow cooker and cook mixture 10 minutes longer. Serve in buns sprinkled lightly with red pepper flakes to taste if desired and optional shredded cheese.