You can make the milk sauce up to one day ahead; keep, covered, in the refrigerator. Whisk the sauce again briefly before spreading it on the sandwich. To make for brunch, put sandwiches together ahead of time, then refrigerate, covered with plastic. Bake them just before serving.
Source: Everyday Food, November 2003
Prep Time 15 minutes
Total Time 40 minutes
3 tablespoons butter
3 tablespoons all-purpose flour
1 cup milk
Pinch of ground nutmeg
Coarse salt and ground pepper
8 slices white sandwich bread
3/4 pound cooked ham, thinly sliced
2 cups shredded Gruyere, or other melting white cheese
Dijon mustard, for serving (optional)
Preheat oven to 400 degrees. Heat butter in a medium saucepan over medium heat until foamy. Add flour; cook, whisking constantly, until smooth, 1 to 2 minutes (do not let color).
Add milk and nutmeg to saucepan. Whisking constantly, bring to a simmer; continue cooking until thickened, 2 to 3 minutes. Season with salt and pepper. Transfer to a shallow dish; cover with plastic wrap, pressing it directly on the surface of the sauce. Refrigerate at least 10 minutes.
Place 4 slices of bread on a baking sheet. Spread each slice with sauce, dividing evenly; layer with ham, and sprinkle each with 1/4 cup cheese. Top with remaining bread; sprinkle with remaining cup cheese. Bake until cheese is melted and golden, about 15 minutes. Serve hot, with mustard on the side, if desired.