Preheat the oven to 350 degrees.
Place the olive oil into a large skillet over medium heat. Add the sliced onions and cook, stirring often, for 20-30 minutes, or until caramelized and golden brown. Season with sea salt, to taste. Side Note: If the skillet gets dry, add a tablespoon of water or two.
Place the sliced focaccia bread into the preheated oven and let the roll warm up, about 3-5 minutes. Slice the leftover roast beef into thin slices. Remove the roll from the oven and spread mayonnaise and horseradish mustard, to taste. Place the roast beef onto the bottom bun then top with a slice of Havarti. Place back into the oven and cook until the cheese melts, about 3-4 minutes.
Remove the sandwich from the oven. Place a large spoonful of caramelized onions on top of the melted cheese then add the arugula and finally place the remaining half of the roll on top. Serve immediately. Enjoy!