In a small bowl, combine the dry brine ingredients; stir to blend. Pat dry roast with paper towel and apply salt mixture all over the meat. Place roast on a wire rack sitting on a plate, wrap with cling film and transfer to the refrigerator for 15 hours.
The next day, take roast out and let the meat come back to room temperature; about 1 ½ to 2 hours. When ready to cook, preheat oven to 350ºF, make a few incisions and insert pieces of garlic. Rub dry mustard over the meat and sprinkle with hot paprika; season with freshly ground black pepper.
Lightly grease a roasting pan with cooking spray and throw in red onions. Place roast on top and pour chicken broth around it. Wrap pan with foil and transfer to preheated oven. Cook roast 25 minutes per pound or until internal temperature reads 145ºF. Remove from oven and transfer onto a cutting board. Wrap loosely with foil and let it rest for 15 minutes before slicing.