In a medium sized mixing bowl, combine the sour cream, mayonnaise, horseradish, apple cider vinegar, mustard powder, onion powder, cayenne, salt, and pepper.
Cover and refrigerate for 30 minutes.
Caramelized onions and prime rib instructions
Cut the extra fat off of the prime rib and cut the meat into slices or cubes.
Season the beef with blackened seasoning, salt and pepper.
Melt the butter in a medium sized cast iron skillet over medium heat.
Add the onions to the skillet and sauté until soft and caramelized.
In the same skillet, lightly sear the prime rib slices just enough to re-warm the beef.
Hoagie roll instructions
Set the broiler on the oven to high.
Cut the (Italian Bread) hoagie rolls in half lengthwise, spread the butter on the cut side of the rolls and sprinkle garlic salt over the butter.
Broil until all the butter is melted and the bread is toasted.
Constructing the sandwiches
Spread the warm prime rib slices onto the hoagie rolls.
Evenly distribute the onions on top of the beef.
Add the blue cheese to each sandwich and top with 3 slices of provolone per sandwich.
Replace the top bun on the sandwich.
Wrap each sandwich in paper towel and microwave for 25-30 seconds each to melt the cheese.
Cut the sandwiches in half and serve with the creamy horseradish sauce in a small ramekin on the side for dipping.