1.Mix the dry rub ingredients together (salt, pepper, garlic powder and chili powder) and sprinkle all over the bottom round. Allow to sit in the fridge for about 30 min - overnight.
2.Grill the bottom round on a smoker over direct heat at 300 degrees until the internal temperature reaches about 135 degrees. If you do not have a smoker you can grill the round on a standard grill by searing it over direct heat and then moving it to indirect heat until medium rare.
3.Wrap the cooked bottom round in foil for about 30 minutes.
4.Slice the meat thin on a meat slicer or with an electric knife.
5.Slice the onions on the meat slicer or with an electric knife.
6.Serve thin sliced pit beef on rolls with sliced onions and horseradish sauce!