If you don't want to do a bechamel sauce, there is a shortcut: mix sour cream with cheese and an egg yolk, put it in the microwave to melt the cheese and there you go, you have your topping ! It takes 3 minutes to do. More importantly: i like to have mozzarella cheese in the topping and comté inside the sandwich. Mozzarella will melt amazingly and comté is for the nutty flavour. But it's up to you, choose the one cheeses you want or have.