Adapted from AllRecipes
In a medium bowl, mix the cream cheese, horseradish, and mustard until well blended.
Spread a thin layer of the cream cheese mixture over each tortilla. Arrange spinach leaves evenly over the tortillas. Layer the roast beef and shredded cheddar on each, dividing evenly between the tortillas. Starting at one end, gently roll up each tortilla into a tight tube. Wrap with plastic wrap to keep the rolls tight. Refrigerate at least 1 hour. They will keep in the fridge for up to 2 days.
Slice each roll into bite-size pieces and arrange on a serving dish. If desired, offer whole-grain mustard as a condiment.