Make a simple béchamel sauce. Using milk, bay leaves and a small onion.
Add the Dijon mustard, a little Worcestershire sauce and the cheddar cheese. It should be a thick creamy sauce with a dark deep flavour. (well that’s how I like it but by all means use less of the mustard)
Season to taste all the time.

Then with your slices of sourdough bread spread a little butter on one side and grill until golden (one side only) 
Turn the bread over and spread each slice with a thin layer of the béchamel sauce and add generous slices of ham. Add your grated gruyère cheese and form Into a sandwich.
In a frying pan melt some butter and fry gently on both sides until golden.
Transfer to a preheated oven and cook for 4-5 minutes so it warms right through to the centre.

Now you are ready to plate up, but like a gentleman I am, I fried a free range egg to turn one of the sandwiches into a croque madame!! Unfortunately or maybe fortunately in this case, I had no female company to share with.....So I ate them both myself 😉😉 All the best and enjoy.