Adding toasted whole-wheat breadcrumbs gives this dish a nice crunch that balances out the creaminess of the sauce. Rather than baking the shells with the crumbs on top (which can make them soggy), toast the breadcrumbs separately, and toss a handful over each serving right before you put it on the table.
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.