Leftover au jus (or two cups beefs stock simmered with two Tbsp flour)
Horseradish Sauce
Instructions
If you have leftover au jus, simply heat it in a small pot over medium-high heat. Reduce the heat to low to keep warm.
Melt butter over medium heat in a saute pan. Add sliced onions and stir into the butter. After a couple minutes the onions will soften slightly. Season with salt and pepper and continue stirring. Continue cooking over medium heat until onions turn a nice, golden brown, 5 minutes. Add ¼ cup of beef stock and allow onions to soak up. Saute an additional 3 minutes and remove from heat.
Place the hoagie buns with the inner side facing up on a baking sheet. Preheat oven broiler to high. When warm, place buns in middle rack and toast, watching carefully, until golden brown. They will go from barely toasted to burnt in a hurry, so watch closely.
Remove buns and turn off broiler. Next, preheat oven to 400°.
To assemble sandwich, split about ⅓ of a pound of leftover prime rib between the top and bottom bun. Top one side with a spoonful of sauteed onions. Top each side with a slice of havarti cheese. Toast open face until cheese melts, about 5 minutes.
Remove pan from oven. spread (or squeeze) horseradish sauce over top of sandwich and press top and bottom together. Serve immediately with warmed au jus in a small side cup for dipping.