Cut each piece of string cheese in half so you have 2 shorter sticks. Working one at a time, make 4 cuts vertically in one end of the cheese sticks, stopping about 3/4 inch from the top. Roll the cheese so the cuts are not parallel to the cutting board, and make 2 or 3 more cuts to form the bristles of the broom. Press a pretzel stick into the uncut end of the cheese stick to make the handle. Tie a chive in the middle of top (uncut) end of the cheese stick; this holds the broom head together. Repeat for the remaining brooms.
Spread the mustard haphazardly on the bottom of a black or white plate. Stand the brooms up on their bristles as if they are sweeping up the mustard.