Step 1

Prepare your horseradish sauce by mixing the sauce ingredients together in a small bowl and refrigerate until needed.

Step 2

Melt 2 tablespoons of butter in a large skillet.

Step 3

When the butter is melted, add the sliced onions and mushrooms to the pan.

Step 4

Saute the onions and mushrooms over high heat until they begin to brown. Let them cook for 3-5 minutes without moving them around, so they have a chance to begin the caramelization process.

Step 5

Turn the mushrooms and onions over and reduce to medium heat and let them continue to cook for another 5-7 minutes. Stir them around during this part of the process..

Step 6

While the mushrooms and onions are cooking, trim the leftover prime rib. Remove the bone and extra fat from the edges of the meat. Take out any fat deposits from the middle of the beef ( I took this picture before removing the piece of fat in the middle)

Step 7

Sliced the baguette in half log ways so you make a top and bottom.

Step 8

Slather a good amount of creamy horseradish sauce on the bottom side of french bread.

Step 9

Add a good handful of arugula (the peppery flavor of arugula improves the overall flavor of the sandwich.

Step 10

Place thinly sliced prime rib on top of the arugula ( I like to fold the meat, setting it side by side so it’s not just lying on top of each other).

Step 11

Add the caramelized onions and mushrooms to the sandwich.*Add a few slices of provolone cheese (or swiss cheese) to the sandwich for additional flavor. If you want melted cheese, add the slices before the warm mushrooms and onions, and the heat will help melt the cheese

Step 12

Cover with the top of the baguette and cut into sandwiches. Serve with hot au jus and extra horseradish sauce.