Chop the Basil leaves to desired size and combine in a small bowl with mayo and horseradish. Whisk together until the sauce is thoroughly mixed.
Toast your bread slices and spread a generous tablespoon of the mayo on one side of each slice. Layer your sandwich from the bottom up starting with the roast beef, havarti, lettuce, tomato, and red onion. Season with a light sprinkle of salt and pepper. Top with other slice of bread and cut in half.