First make the bechamel sauce: boil the milk with the bay leaf, remove from the stove and leave it for 20 min. Melt the butter. Add the flour and stir for 2 min.
Remove from the heat and add the milk slowly. Return to the stove and stir for several min. to obtain a creamy mixture.
Arrange a layer of lasagna sheets on top, sprinkle with 1/3 Bolognese sauce, then 1/3 of the bechamel. Put another layer of sheets, then the remaining Bolognese and 1/3 bechamel.
Finish off with the rest of the sheets and the remaining bechamel, sprinkle with parmesan. Bake in a preheated 428°F (220 °C) oven for 20 min.