Preheat oven to 350°.
Carefully slice the croissants.
Place them on a parchment paper-covered baking pan or cookie sheet.
Generously slather the mustard mayonnaise sauce on one-half of the croissants.
Fold a couple of slices of deli ham and place them on one-half of the croissants.
Place a slice of cheese on top of the ham.
Slather any remaining mustard mayonnaise sauce and the second half of each croissant.
Place the tops on the croissants.
Melt the butter in the microwave for 20-30 seconds.
Liberally brush the top of each croissant with the butter.
Sprinkle the sesame seeds on top of the buttered croissants.
Lightly cover the baking pan with alumnium foil.
Bake for 10 minutes.
Remove the foil and bake for another 5 minutes.