Pull back the fat cap from the roast and brush the meat all over with some of the stock.
Coat the meat all over with half of the pickling spice and rub it inches.
Combine the black pepper and garlic powder with the salt, then divide this mixture in half and sprinkle half of it over the meat.
Leave the onions in whole rings and layer them over the meat.
Return the fat cap to its original place.
Tie the roast back together with kitchen twine at 2-inch intervals.
Place the meat in a large, deep roasting pan, making sure that fat cap is facing up. Pour the remaining stock into the pan.
Season the outside of the meat with the remaining pickling spice and remaining spice-salt mixture.
Roast for 20-25 minutes (at 450-degrees Fahrenheit). Reduce oven temperature to 350 degrees F and roast for an additional 1 hour 15 minutes - 1 1/2 hours or until a meat thermometer reads 130-degrees F for rare or 140-degrees F for medium-rare or longer if you prefer, but be careful not to over-cook so it won't be over-done and dry. (The best way to acquire perfect results is by using a reliable meat thermometer).
Remove the meat from the oven and let sit in a warm place for 20 minutes before slicing (this will help the beef retain its juices, therefore resulting in juicy/moist meat. If it is sliced right away, the juices are released prematurely).
Slice the roast, discarding the fat cap, and serve.