You can make the bechamel sauce three days ahead (let it cool completely before refrigerating). The onions can be cooked, peeled, and refrigerated overnight. Heat the onions and sauce together over low heat before serving.
A blue cheese bechamel makes this tart deliciously rich, so you need only a small piece. It's a good starter or side dish for a holiday meal, but it works just as well for lunch or a light dinner with a green salad.