This sandwich involves several different elements, one of which is a homemade béchamel sauce made from butter, flour, and milk and flavored with salt, nutmeg, and white pepper.
Preheat the oven to 350 F.
To make the béchamel sauce, melt 2 tablespoons butter in a saucepan over medium heat.
Add the flour to the butter and cook it, stirring constantly, for about 30 seconds until a bubbly paste forms.
Pour in the milk and stir the mixture until no flour lumps remain.
Add ¼ teaspoon salt, along with the nutmeg and white pepper, and cook the mixture for 3-4 minutes, stirring until it thickens. Remove from the heat and aside.
In a large pan, melt another tablespoon of butter over medium heat and fry the onion for a few minutes until it turns golden and softens.
Add the mushrooms, black pepper, and remaining salt and cook for 4-5 minutes, stirring frequently, until the mushrooms are soft. Set the mushroom filling aside.
To assemble the sandwiches, spread mustard and béchamel on 1 side of all 4 slices of bread. (You’ll use up all of the mustard, but not all of the béchamel sauce.)
Top 2 of the bread slices with 2 slices of cheese each, followed by the mushroom filling and remaining cheese. Close the sandwiches with the other bread slices, with the condiment side facing inward.
Melt the rest of the butter in a pan over medium heat and fry the sandwiches on each side until golden brown.
Put the sandwiches into an oven-safe pan, then spread the rest of the béchamel sauce on top and sprinkle on the parmesan.
Bake the sandwiches for 5-7 minutes until the cheese melts, then allow to cool for 3-5 minutes before serving so the sauce doesn’t spill out.
