First you need to make the M&B cheese sauce.

1. In a small saucepan on a medium heat melt the butter
2. Add the flour and stir for two minutes until bubbling and smooth.
3. Add all the milk and keep mixing until thick and smooth like custard.
4. Add mustard, gruyere and nutmeg until melted into a lovely cheese sauce. Let cool.

To make the sandwiches

1. Lay out your sandwich as two slices in a board. Spread the sauce on one side nice and thick.
2. Add half the grated cheese mix and a sprinkling on the onions.
3. Put the other slice on top to make a sandwich and press down firmly
4. heat a frying pan on a medium heat big enough for the sandwich and add a knob of butter. Once bubbling and melted add the sandwich and put another frying pan or plate on top to press it down for 3 minutes or so.
5. Flip the sandwich so the newly golden underside is now facing up. Plate or pan back on top for a further 3 minutes.
6. gently open the sandwich and check all cheese has melted, before remiveing to a board, allowing to cool for about a minute and then cutting in half and serve with a pickle.