Step 1

To caramelize onions, melt butter in skillet over medium high heat. Add sliced onions, arrange in single layer, and don't stir until they start to brown. Gradually reduce heat to prevent burning and only stir when they start to brown too much. Continue cooking until a dark caramel color forms. Set aside.

Step 2

To make spread, combine mayonnaise and horseradish. I chose to set my roasted garlic cloves directly on the sandwich but you can mash and mix with the horseradish mayo, if you prefer.

Step 3

In small bowl, dress arugla greens with olive oil and vinegar. Season with salt and pepper, if desired.

Step 4

To serve, layer sandwich with spread, dressed arugula, prime rib and caramelized onions. Serve cold.