Toast the 4 slices of sourdough bread and set aside.
In a small bowl, mix together the mayonnaise, horseradish, lemon zest, and a sprinkle each of salt and pepper. Spread generously on each piece of bread.
Choose 2 pieces of bread to be the base of each sandwich. Layer on the roast beef, divided evenly between the two sandwiches. Top with slices of cucumber and red onion, layer two slices of Muenster on each sandwich, and pile with herbed lettuce mix.
Close each sandwich with a second piece of bread, cut in half if desired, and serve.