Take one bite of this leftover prime rib sandwich and you will be looking for excuses to make more prime rib roasts.

I love this mouthwatering sandwich because not only does it taste incredible and use basic ingredients, it is definitely my favorite way to repurpose prime rib leftovers. However, best of all, it is actually a great recipe to use with any deli roast beef. If you are looking for a gourmet deli sandwich that will make your eyes roll back in your head, this is it!

What is it?

This leftover prime rib sandwich recipe features thinly sliced prime rib, arugula, and sweet caramelized onions, piled high on a hoagie roll with a homemade creamy horseradish sauce. This tantalizing sandwich can be enjoyed warmed or cold.

Ingredients and Substitutions

• Leftover prime rib - This sandwich can also be made with leftover steak or any high-quality deli roast beef. I specifically recommend using roast beef that is pink in the center.
• Sweet onion - Yellow onions are good substitutes.
• Butter - You can substitute olive oil.
• Baby arugula - Arugula has a lot more flavor, but baby spinach or any spring greens can be used as well.
• Hoagie rolls - I like a soft bakery fresh roll to pair with the tender cut of beef, but crispy bread like a crusty baguette can be used too.
• Mayonnaise
• Prepared horseradish
• Dijon mustard
• Garlic cloves
• Fresh ground black pepper

How to make it

You will need to thinly slice your leftover prime rib roast for these sandwiches. This is best done when the meat is cold the next day. However, if desired, there is nothing wrong with making this with thicker cuts of meat too.

The most involved step in this recipe is caramelizing the onions. It is important not to rush this step or you will be missing out on a lot of great flavor. Melt butter in a hot skillet set over medium heat and add sliced onions. Saute them for thirty minutes, or until brown and sticky. If you rush this step you will not have caramelized onions, you will just have sauteed onions.

While the onions caramelize you can mix together the mayo, horseradish, Dijon mustard, minced garlic, and black pepper in a small bowl, then set it aside.

I also highly recommend taking the time to toast the interior of your hoagie rolls. This will improve the texture of your sandwich. The easiest way to toast the roll is to place it face down in the hot skillet when the onions are done caramelizing (although you can certainly use a separate large skillet). It will only take a minute or two to turn golden brown. Alternatively, you can set the buns on a baking sheet and toast them under your broiler.

Assemble your sandwich by adding the arugula to the hoagie bun, topped with prime rib, caramelized onion, and your homemade horseradish mayo sauce. Serve immediately.

Recipe Tips & Tricks

• This sandwich can be served with cold meat or warmed slices. If you wish to warm the tender meat it needs to be done carefully so you do not cook the red out of it. The best way to do it is to warm au jus or beef broth in a saucepan to 120-130 degrees and dunk the thin slices of prime rib into the liquid to warm it.
• If you wish to add a slice of cheese, Provolone cheese would be my top recommendation, followed by blue cheese.
• You can add fresh herbs like thyme to the creamy horseradish sauce.
• Sun-dried tomatoes would also be a great topping.

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