Béchamel is the simplest of French mother sauces. The ingredients are flour, butter, milk, salt and pepper (preferably white pepper). The Joy of Cooking gets fancy by simmering the milk with an onion stuck with cloves and a bay leaf, which are all discarded before adding the milk to the butter/flour roux; Julia Child gives the simpler recipe. Though sometimes called a cream sauce, proper béchamel doesn't have any cream unless you count the butter, but cream can be added to make a richer white cream sauce.
A cream soup, such as cream of celery or chicken, starts out as a béchamel to which stock and the veggies or meat are added. A thicker version with grated cheese added, as PP indicated, can be used to make a mac and cheese that beats anything out of a box.
When I was a teen, we took a vacation that included a week outside of
Disneyland. We'd travel to see other sights in LA, but DL was our base, and the having paid for a single day's admission, adding extra days was cheap. One of the restaurants there (Carnation?) had a whitefish in béchamel that was wonderful. Since we didn't eat meat unless it was kosher, and my mother never made rich sauces or gravies, I went back to that restaurant several times for the fish. It's one of the strongest memories I have of that trip.
Try it, you'll like it.
