Preheat oven to 325 °F and place roast upright in a roasting pan.
Stir mustard, horseradish, salt and pepper and rub over surface of roast.
Place uncovered in centre rack of oven and cook for 2 to 2 ½ hours to an internal temperature of 130 °F for rare.
Remove roast from pan to a cutting board and rest, tented with foil, for at least 15 minutes before carving.
To carve, use a sharp knife to carefully slice down the bones to remove. Carve thin or thick slices of beef, as desired.
Use the roasting pan from the prime rib roast and drain all but 2 tbsp drippings.
Over medium heat, sauté mushrooms in the drippings until soft, about 8 minutes.
Add flour and cook over medium heat 3 minutes, until flour is worked into a paste.
Add stock gradually, whisking in slowly to avoid lumps, stir in thyme then bring to a full simmer.
Add brandy, simmer 2 minutes more, then add cream. Return to a simmer then season to taste and serve.
Preheat oven to 350 °F.
Toss potatoes with a touch of olive oil to coat, season lightly and add rosemary.
Roast in a baking dish, covered for 20 minutes, then uncover and cook until fork-tender, about 25 more minutes, stirring occasionally.