Use a mandolin slicer to get all the veggies paper thin; that way they'll melt into one another, and this whole mixture will become homogenized goodness.
Don't let the bechamel intimidate you. This French Mother Sauce is an alchemy of very simple ingredients that become a tremendous sauce through the proper application of heat. Just don't blast either the flour mixture, or the milk, with heat higher than medium, and you should be good. TAKE YOUR TIME, and don't pump up the heat to speed it up. Your patience will be rewarded with a luscious velvety sauce.