In a medium saucepan, melt 4 tablespoons of unsalted butter over medium heat. Whisk in ¼ cup of all-purpose flour, stirring continuously for about 2 minutes to create a light roux.

Gradually pour in 2 cups of whole milk while whisking vigorously to prevent lumps. Continue to cook for 5-7 minutes until the sauce thickens and coats the back of a spoon. Sprinkle in a pinch of ground nutmeg and white pepper, then set aside.

Preheat your oven to 375°F (190°C). Prepare your baking sheet by lining it with parchment paper.

Take 8 slices of brioche bread and lay them out on a clean surface. Spread a thin layer of Dijon mustard on one side of each slice.

Layer 4 ounces of Black Forest ham followed by a generous sprinkle of 1 cup of grated Gruyère and ½ cup of Swiss cheese on half of the brioche slices. Place the remaining slices on top, mustard-side down, to create sandwiches.

Spread a generous amount of the prepared Béchamel sauce over the top of each sandwich. Sprinkle the remaining Gruyère cheese over the sauce.

Arrange the sandwiches on the prepared baking sheet and bake for 15-20 minutes until golden brown and bubbling.

Optionally, broil the Croque Monsieur Appetizers for an additional 1-2 minutes for extra crispiness. Keep a close eye to avoid burning.

Let the sandwiches cool for a few minutes before garnishing with chopped fresh chives if desired.