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First, make a perfect béchamel sauce. Put the milk, cream, garlic, bay leaves and onion in a small pan over a medium heat and stir together. Heat until nearly boiling, then turn the heat off and leave for 10 mins to infuse. Pass the liquid through a sieve and discard the bay and onion.
Melt the butter with the flour in another pan and cook over a gentle heat until it smells biscuity. Gradually whisk in the warm milk and bring to a gentle boil, stirring regularly, then add the Dijon mustard, a little grating of fresh nutmeg and some seasoning. It should be a lovely thick creamy sauce with a deep flavour.
Brush the slices of bread with melted butter. Place them on a baking tray and toast one side under a grill, buttered-side up, until golden. Take them out and heat oven to 220C/200C fan/gas 7.
Turn the bread over and spread each slice with a thin layer of Dijon, followed by a layer of the béchamel. Cover the sauce with grated gruyère, and then a slice of ham. Form the slices into two sandwiches, spread a thin layer of the sauce over the top slice and sprinkle more gruyère on top. Put the sandwiches in the oven and bake for 10-15 mins or until golden. Mix the mustard and mayo together and serve alongside.