Meanwhile, make the sauce by melting the butter in a tall saucepan. Remove from heat and stir in the flour. Place back on the stove and cook for 1 minute, taking care not to burn the mixture. Gradually whisk or stir in the milk and cook till the sauce thickens, for around 7-8 minutes. Stir constantly, or the sauce will stick or turn lumpy. When smooth and thickened, remove from heat and add most of cheese, reserving a good handful for the topping.