Preheat oven to 400 degrees and line a baking sheet with parchment paper or a silicone mat.
Unfold puff pastry out onto a cutting board and smooth out any creases.
Using a pizza cutter or a sharp knife, cut the rectangle lengthwise three equal strips. Cut these strips diagonally into triangles. You should end up with 6 long, narrow triangles.
Place a slice of cheese on top of a slice of ham (if your cheese is longer in width and hanging outside the ham a lot, you may want to trim it) and fold over. Place them at the wide end of each triangle.
Starting from the wide end where you’ve placed the ham and cheese, begin rolling the dough towards the pointy end of the triangle. Ham and cheese can be hanging out of the ends a bit and you can leave the dough open at the ends.
Place formed croissants on prepared baking sheet and brush each one lightly with egg wash (this will help the croissants get glossy and golden). Bake for 14-18 minutes, until croissants are puffy and golden.