Set roast out on the counter about 1 hour before grilling. Pat dry with paper towels and lay on a baking sheet.
Drizzle with olive oil, rubbing on all sides.
Mix brown sugar, garlic, smoked paprika, kosher salt, pepper, dry mustard and red pepper flakes.
Cover entire roast with dry rub, pressing it in to coat.
Prepare a grill fire to 300° with hickory or oak for smoke flavor.
Place roast over indirect fire and cook with the grill lid closed for 1 1/2 hours or until roast reaches desired doneness. Use a digital thermometer for accurate results. For a medium roast, cook to 135°.
Remove from grill and allow to rest 15 minutes before slicing.
To make sandwiches, layer buns with horseradish sauce, tomato, sliced roast beef and top with caramelized onions.