Trim the steam top and bottom, then cut them into lengths. Parboil in well salted water until tender, about 5-10 minutes depending on their size. Drain and let cool.
Meanwhile, prepare the béchamel sauce following the directions in this post.
Grease the bottom of a baking dish large enough to hold all the chard stems in one or two layers (or, if you prefer, individual dishes as pictured here). Lay on a layer of chard stems, topped with slices of ham and/or mozzarella if using, then nap with béchamel and top everything with a good dusting of Parmesan. Repeat until the chard is used up. Top with a sprinkling of breadcrumbs, if using. Finally, dot the top with bits of butter.
Bake in a moderately hot (190C/375F) oven for about 20-30 minutes, or until the sauce is bubbling and the top is nicely brown.
Remove from the oven and let cool for about 10-15 minutes before serving.