Cut the croissants in half lengthwise, then brush the base with butter. Save for later.
Heat the ham slices for about 10 minutes in total in a large skillet over medium heat. Proceed in two steps.
Brush the inside of the croissants with Dijon mustard. Cover with the cheese and ham slices, then garnish with small sliced pickles. Cover with the top of the croissants (brushed or not with Dijon mustard) and serve immediately with a green salad.