Gruyere Croque monsieur

Melt butter in a small saucepan over a medium heat. Add flour and cook for 2 minutes, stirring. Remove pan from heat and gradually add milk, whisking until combined. Return pan to a medium-low heat and bring mixture to the boil, stirring constantly. Simmer for 2 minutes. Remove pan from heat and add gruyere, stirring until smooth and combined.

Spread extra butter on 1 side of each bread slice. Put 4 slices, butter-side down, on a breadboard. Spread with mustard. Divide ham and sauce between bread slices. Sandwich with remaining slices, butter-side up.

Put a non-stick frying pan over a medium heat. Add sandwiches, in batches, and cook for 5 minutes on each side or until golden. Slice each sandwich in half to serve.