For the bread, bring the milk to the boil then remove and tip into a large bowl. Add the water and leave to cool until slightly warm. Chuck the yeast in, mix briefly and leave to bloom for a few minutes.
In a separate bowl, combine your salt, sugar and bread flour. Tip this into the yeast-y mix via a sieve. Add your butter. Mix together until just combined, then cover with a damp cloth and leave to hang out for 10 minutes or so.
Oil a surface and knead this mix together about 5 or 6 times. Put back into your bowl, cover and leave to rise for an hour, kneading briefly on the same surface from time to time.
Once double in size and looking good- bring out and knead one last time before putting into greased, lined dutch oven or spring-form tin. Leave to rise under a damp cloth for a further 20/30 minutes. Dust with flour then bake at 160°C for 50 minutes. Tip onto a rack and leave to cool.
Meanwhile, make your bechamel. Foam your butter over low-medium heat then tip in the flour. Cook, stirring frequently, for about a minute, then gradually add all the milk, whisking continuously to prevent lumps from forming. Season with salt, pepper and nutmeg. Bring to a simmer and allow to thicken well, stirring from time to time.
Turn the heat down to very low, or off even, then add the cup of gruyere in two stages, mixing between additions. Leave to cool completely, covered, until spreadable and thick.
Cut your loaf of bread into 3 discs. Butter well and grill until nicely toasted.
Put the bottom disc down on a baking sheet. Spread lightly with dijon mustard. Top with a layer of cheesy bechamel, then ham and cheese. Top with your middle disc, the bottom side of which you have spread with another layer of bechamel. Repeat the process again.
Spread the top of your loaf with a little mustard then top with the remaining bechamel and some additional gruyere cheese.
Bake for 40 minutes at 160°C, or until the top is nicely coloured and the inside is hot, cheesy and creamy.