In a bowl, mix together cream cheese, horseradish, and Dijon mustard.
Assemble pinwheels by spreading a thin layer of the cream cheese mixture over each tortilla. Then layer spinach, roast beef, and cheese. Tightly roll up each tortilla and wrap it with the foil or plastic wrap.
To serve, slice each roll into one-inch slices.
You can store them up to two days.