Warm 150ml/5fl oz of the milk in a small pan to blood temperature then mix in the yeast and set it aside until the mixture becomes frothy.
Sift the flour and salt into a bowl and make a well in the centre. Add the yeast mixture, the butter, Gruyère and the remaining milk and knead the mixture for about 10 minutes, adding a little more flour if needed.
Place the dough in a large, clean bowl, cover with a dry tea towel and set aside in a warm place until it has doubled in size – about one and a half hours.
Remove the dough from the bowl, divide it into six equal portions and roll each into a long strand. Pinch the strands together at the top, plait together loosely and place on a greased baking tray.
Brush the loaf with the beaten egg and place in a cold oven. Turn the oven to 170C/325F/Gas 3 and bake the loaf for 45-50 minutes or until the loaf is golden-brown and the bottom sounds hollow when its tapped.