This Leftover Prime Rib Sandwich is loaded with prime rib and topped with warm caramelized onions and mushrooms. It's a delicious way to make use of the prime rib leftovers.
Sauté the onions and mushrooms over high heat for 3-5 minutes without stirring. Turn the mushrooms and onions over and let it cook for 5-7 more minutes, stirring constantly.
Slather a good amount of creamy horseradish sauce on the French bread. Add a good handful of arugula. Place thinly sliced prime rib on top of the arugula, then the caramelized onions and mushrooms.