This is flavor town between two slices.
Mix together the mayonnaise and horseradish then add salt and pepper to taste. Set aside.
Toast the bread and then spread the mayonnaise mixture on one side of each piece of toast.
On one of the pieces of toast with the mayo side up place the remaining ingredients in order, spinach, roast beef, cheese, onion rings, and tomatoes then top with the other slice mayo side down.
Cut sandwich diagonally and serve with chips and a pickle.