Place one oven rack in the center of the oven, and another closer to the broiler element. Preheat your oven to 425°F (220°C). Prepare a small sheet pan lined with foil or parchment. Make the bechamel sauce by melting the butter in a small saucepan over medium heat, then whisking in the flour. Allow it to cook for 2-3 minutes, then gradually whisk in the milk until the mixture just begins to bubble and thicken. Remove from heat, and whisk in the Dijon mustard, pepper, and nutmeg. Set aside.You can store any extra bechamel sauce in an airtight container in the refrigerator for several days to enjoy croque-monsieurs all week!
Step 2
Lay two slices of bread on your prepared sheet pan. Spread each slice generously with bechamel sauce. Top with sliced meat, and sprinkle each with ¼ of the shredded Gruyère. Add the top slice to each, and cover generously with bechamel, going all the way to the sides (this will prevent the edges from burning). Divide the remaining Gruyère over each sandwich, and top with the grated Parmigiano-Reggiano cheese.
Step 3
Place in the middle rack of the oven and bake for 6 minutes. Turn on the broiler, and broil for 2-3 minutes, or until golden brown on the top, to finish. (If you have used parchment paper, be careful not to allow the parchment to get too close to the broiler). Serve immediately.