2 tablespoons unsalted butter
Preheat oven to 400*F.
Melt butter over low heat in a small saucepan. Add the flour, stir with a wooden spoon for 2 minutes. Slowly add the hot milk, whisking constantly, until thickened. Remove from heat and whisk in salt, pepper, nutmeg, 1/2 cup Gruyere, and the Parmesan.
Toast bread (either in a toaster, or by placing on a baking sheet for 5 minutes in the oven, then flip slices over and bake 2 more minutes).
Spread a thin layer of mustard on half the toasts, then a slice of ham, and a sprinkle of Gruyere. Top each with a plain toast and slather the sandwiches with the cheese sauce. Sprinkle the sandwiches with the remaining Gruyere. Bake for 5 minutes, then place under the broiler for 3-5 minutes, until bubbly and lightly browned.
Serve immediately. (Serves 4-8)
adapted from Barefoot in Paris
*I find ham to be extremely salty, so a teaspoon of salt in the cheese sauce was too much for me, especially considering Parmesan is a salty cheese. Use caution when salting the sauce!
**Ina recommends Pepperidge Farm sandwich bread for this recipe.