Preheat your oven to 400°F (200°C).

In a small saucepan over medium heat, melt 2 tablespoons of butter. Once melted, add the flour and whisk continuously for 1-2 minutes to make a roux.

Gradually add the milk while whisking to avoid lumps. Continue to cook, stirring frequently, until the sauce thickens (about 5-7 minutes).

Once thickened, remove from heat and stir in the Dijon mustard, salt, and pepper to taste.

Spread butter on one side of each slice of bread. On the unbuttered side, place a slice of ham followed by a slice of Gruyère cheese. Top with another slice of bread, buttered side facing out. Repeat for the remaining sandwiches.

In a large skillet over medium heat, cook the sandwiches for about 3-4 minutes on each side, or until golden brown and the cheese begins to melt.

Once all sandwiches are cooked, place them on a baking sheet. Pour the prepared béchamel sauce over the top of each sandwich, then sprinkle additional Gruyère cheese on top if desired.

Bake in the preheated oven for 5-10 minutes, or until the cheese is bubbly and golden brown.

Remove from the oven and let cool slightly. Garnish with fresh parsley if using, then slice in half and serve immediately.