Steps
1. Preheat the oven to 180°C (350°F).
2. In a small saucepan, melt the butter over medium heat. Add the flour and cook for 1-2 minutes, stirring constantly.
3. Gradually whisk in the milk until smooth. Cook the sauce for 2-3 minutes, or until it thickens. Season with salt, pepper, and nutmeg (if desired).
4. Toast the bread slices lightly and place them on a baking sheet.
5. Spread a layer of the sauce on one side of each bread slice. Top one slice with the sliced chicken and sprinkle with half of the grated Gruyère cheese.
6. Place the other bread slice on top, sauce side down. Spread the remaining sauce on top of the sandwich and sprinkle with the remaining cheese.
7. Bake in the preheated oven for 10-15 minutes, or until the cheese is melted and golden.
8. Serve hot and enjoy!