If you want your prime rib warm in your sandwich you can reheat it carefully in a 200-degree oven wrapped in foil in a covered baking dish for about 10 minutes. Whatever you do...DO NOT microwave your prime rib! You will ruin the taste and texture. Trust me... it will be a rubbery disaster! You have spent the money on a premium piece of meat and the time to ensure it's cooked to perfection don't take any chances! I prefer instead to serve the prime rib at room temperature in my sandwiches with piping hot au jus for dipping!