2 large white onions, cut in 1-inch (2.5 cm) slices
1/4 cup (60 mL)Dijon mustard
1 standing prime rib roast (about 6 lb/2.7 kg)
1 1/2 cups (375 mL)kosher salt
1/3 cup (75 mL)prepared horseradish
2 tbsp (30 mL)mustard seeds
2 tbsp (30 mL)thyme leaves
2 tbsp (30 mL)rosemary leaves
2 tbsp (30 mL)cracked black pepper
5 bay leaves, crumbled
3 egg whites, whipped until frothy
Step 1 Preheat oven to 450°F (230°C).
Step 2 Layer onion slices in a roasting pan. Set aside.
Step 3 Slather mustard evenly over prime rib. Set aside.
Step 4 In a large bowl, combine salt, horseradish, mustard seeds, thyme, rosemary, pepper and bay leaves. Add egg whites and stir until mixture resembles wet sand. Press salt mixture evenly onto fatty side and ends of prime rib, then place bone side down on onions in roasting pan.
Step 5 Roast for 15 minutes. Reduce oven temperature to 325°F (160°C) and continue roasting for 20 minutes per pound (450 g) or until instant-read thermometer inserted into thickest part of roast registers 130°F (55°C) for medium-rare doneness.
Step 6 Transfer prime rib to a cutting board and let rest for 20 minutes. Remove and discard salt crust before slicing meat.