Cut the chicken fillet into steaks, season with salt, black pepper, soy sauce and let them sit for 15-20 min. in the fridge.
Meanwhile, peel and cut the potatoes into slices. Arrange the potatoes in an oven-safe tray, put the steaks on top, add a little water and bake about 30 min.
In the meantime, make the bechamel sauce from the butter, flour and milk. Pour it over the steaks on top, grate on the cheese and bake another 10 min.