Slice the chilled steak as thinly as possible across the grain.
Stir the sour cream, horseradish and Dijon together until smooth.
Toast the ciabatta rolls cut-side down in a dry skillet for 2 to 3 minutes until lightly crisp.
Add the olive oil and steak to the skillet, season with salt and black pepper, and turn for 2 to 3 minutes until reheated to 165°F without hard browning.
Spread the horseradish cream over both cut sides of each roll.
Divide the warm steak and arugula among the rolls and close them while the meat is still warm.