BECHAMEL SAUCE: In a medium-sized saucepan, melt butter over medium heat. Stir in flour and cook for just up to 2 minutes, before it becomes brown. Gradually pour hot milk in it and keep stirring. Bring to a boil and cook it until it thickens to your liking, about 2 more minutes. Add lemon juice. Season with salt, black pepper and nutmeg. Preheat the oven to 180 °C or 350 °F.