Bring a pot of water to a boil, add salt, to taste. Blanch the spinach in the water for a few seconds, then transfer spinach in a strainer to a bowl of iced water to stop the cooking. Squeeze excess water out of the spinach, and chop roughly.
Heat butter in a saucepan. Add the flour and cook into a light tan color, making a blonde roux. Add the onions and cook for 1 minute, then add the milk, nutmeg, salt, and pepper, and whisk to remove any lumps from the roux. Bring to a simmer and cook for about 6 minutes.
Combine the spinach, bechamel, and adjust seasonings. Fill 6 to 8 (4-ounce) gratin dishes with the creamed spinach. Mix the cheese and breadcrumbs, and scatter over top of gratin. Put the gratins in the oven, and cook until bubbling and golden on top, about 25 to 30 mintues.