Spread each slice of ham with a little mustard.
Place a slice of cheese on each slice of ham and roll from the short end so that the cheese is completely enclosed in the ham.
Cut roll-ups in quarters crosswise and secure loose ends of ham with wooden picks.
Arrange 20 roll-ups in a single layer on a heat-resistant, non-metallic serving platter.
Heat, uncovered, on full power 3 to 4 minutes until cheese begins to melt and ham is hot.
Heat remaining roil-ups.