Rub the rib roast generously on every surface with the coffee rub. Let stand at room temperature for one to two hours in order to promote more even cooking. Alternatively, leave uncovered and return to refrigerator overnight. About 3 hours before you plan to cook the beef, remove it from the refrigerator.
While the roast tempers, preheat oven to 400°F. Place the potatoes on a baking sheet and season with cooking oil, salt and pepper. Bake until tender, about 50 minutes. Remove from heat and allow to cool.
Slice each cooked potato in half length-wise and scoop the flesh into a large bowl, keeping the skins intact. Place the skins on a baking sheet with the cut side facing up. Mix the remaining ingredients with the scooped potato in the bowl until combined. Spoon the mixture back into the potato skins and reserve. This can be done up to one day ahead and the prepared potatoes can be refrigerated in an air-tight container until it is time to reheat.
Place the prepared prime rib on a roasting rack with the fat facing up and the ribs facing down. Roast for 30 minutes at 400°F, then reduce heat to 250°F and cook for an additional 1 – 2 hours, or until you have reached desired doneness. Remove from heat and rest for 20 – 30 minutes prior to slicing.
While the beef cooks, bring a large pot of salted water to a boil. Cook the trimmed green beans just until tender, about 30 seconds. Transfer immediately to a bowl of ice water.
Drain beans and combine with mushroom cream sauce and vegetable stock or cream. Pour into an 8-inch casserole dish and top with cheese and fried onions.
After removing prime rib from oven to rest, turn temperature up to 375°F.
Bake casserole for 20 minutes, or until bubbly and cheese is melted. Bake prepared potatoes for 20 minutes, or until golden brown. Top with fresh chives. These should both be finishing just as the prime rib is rested and ready to slice.