Melt the butter in a saucepan, stir in the flour to make a smooth paste and cook, stirring for 3 min over a low heat without allowing the paste to brown. Remove from the heat and add the milk with the bay leaves or bouquet garni gradually, stirring after each addition until smooth, using a whisk. Return to heat, add the nutmeg and cook for 5 min on a very low heat, stirring all the time until sauce thickens enough to cover the back of a wooden spoon. Remove from the heat and when using for a gratin, add 80g grated cheese of your choice. Mix well and pour over the vegetables.
Steam or boil the vegetables seperately, but do not overcook. Add salt as necessary. Place in an oven dish one next to the other. Pour the bechamel over and bake in a moderately hot oven until the top is golden, for approx 20 min.