1-Melt the butter in a skillet over medium-high heat and add the sliced onions in a single layer. Cook without stirring until they begin to brown, then lower the heat gradually to avoid burning, stirring only as needed until the onions turn a dark caramel color. Set them aside once done.

2-Mix the mayonnaise with creamy horseradish to create a tasty spread. You can add the roasted garlic cloves directly or mash them into the mix for extra flavor in your prime rib sandwich.

3-In a small bowl, dress the arugula with olive oil and white balsamic vinegar or lemon juice. Season with salt and pepper if you like, to add a fresh crunch.

4-Assemble the sandwich by layering the horseradish mayo spread on the bread, followed by the dressed arugula, thinly sliced prime rib, and caramelized onions. Serve it cold to keep everything fresh and flavorful.