Combine the horseradish and mayonnaise in a small bowl.
Place the shredded cheese in another small bowl and toss it with the cornstarch. Add the evaporated milk and heat it in the microwave for about a minute. Stir until smooth.
Melt the butter in a skillet and add the roast beef. Cook it for a few minutes until it's warmed through.
Divide the horseradish mayonnaise among the buns. Top with the roast beef and cheese sauce. Serve warm.
Notes
For the best results, shred block cheddar cheese for this recipe instead of using pre-shredded cheese, which has stabilizers that can cause issues with the sauce.