In a medium saucepan over low heat, melt 3 tablespoons of butter. Whisk in 3 tablespoons of flour to create a roux, stirring constantly for about 1-2 minutes until it’s lightly golden. Gradually add 1 cup of milk, whisking until the sauce thickens and becomes smooth, approximately 5-7 minutes. Season with a pinch of nutmeg, salt, and pepper to taste.

Take two slices of your chosen bread and spread a generous layer of béchamel sauce on one side of each slice. On one slice, layer slices of ham followed by a generous layer of grated gruyère cheese. Top with the second slice of bread, béchamel side down, pressing gently to secure the filling together.

Spread a thin layer of butter on the outside of the sandwich to ensure it toasts beautifully. Heat a skillet over medium heat for about 2 minutes. Carefully place the sandwich in the skillet, making sure it sizzles gently as it starts to cook.

Cook the sandwich for about 3-4 minutes on one side, pressing down with a spatula until the bread is golden brown and the cheese starts to melt. Flip the sandwich and toast the other side for an additional 3-4 minutes.

For extra crispiness, preheat your broiler. Once both sides are golden and the cheese is melty, transfer the sandwich to a baking sheet and place it under the broiler for about 1-2 minutes until the top is bubbly and crispy.

Remove the Croque Monsieur from the skillet and let it rest for a minute before cutting it in half. Serve warm, ideally paired with a crisp green salad or a cup of tomato soup.