Preheat oven 450°F (230°C).
Place the onion slices in the bottom of a roasting pan. Put aside.
Evenly brush the prime rib with mustard. Put aside.
In a large bowl, combine salt, horseradish, mustard seeds, thyme, rosemary, pepper and bay leaves. Add the egg whites and stir until the mixture looks like wet sand. Press the salt mixture evenly over the fat side and ends of the prime rib, then place the cut of meat, bone side down, over the onions in the roasting pan.
Roast for 15 minutes. Reduce oven temperature to 325°F (160°C) and continue cooking for 20 minutes per pound (450 g) or until an instant-read thermometer inserted into the thickest part of the roast indicates 130°F (55°C), so meat is cooked just through medium rare.
Transfer the rib roast to a cutting board and let rest for 20 minutes. Remove and discard salt crust before slicing meat.
Enjoy !