We were looking for a truly impressive version of this French bistro favorite—one with crisp, buttered bread; salty-sweet ham; a creamy white sauce; and plenty of nutty Gruyère cheese. We chose to amp up the béchamel sauce called for in traditional recipes by adding some Gruyère and Parmesan cheeses. To ensure that the sandwiches were ready to eat at the same time, we assembled all four of them on a baking sheet, layering in the cheese sauce and Black Forest ham before topping them all with more sauce and cheese. A trip under the oven’s broiler was all that was needed to meld the sandwich ingredients together, creating a bubbly-browned top in the process. For the best results, be sure to use a good-quality Gruyère here.