Warm a pan over medium heat, and then melt the remaining two tablespoons of butter on low heat.
Once it is melted and bubbly, add the flour and whisk constantly until the raw smell of the flour disappears and the mixture turns beige, like the colour of peanut butter, about 2-3 minutes. The whisking is important so that the sauce does not catch at the bottom of the pan.
Slowly stream in the milk, whisking while you pour.
Continue whisking until no more clumps of roux remain.
Season the sauce with salt and pepper, keeping in mind that the zucchini has already been seasoned.
Continue whisking over medium heat until the sauce thickens, ie about 5 minutes. The sauce needs to be thick, or else the gratin could become mushy.
Once thick, pour the sauce over the zucchini in the greased casserole, stir it in.
Check for the seasoning and if required add a little more salt and pepper and mix it in well.
Sprinkle the grated Parmesan cheese evenly on top.
Bake zucchini au gratin for 20 minutes or until it is bubbly and the top is golden.
Let it cool for 10 minutes before serving.
Serve with warm crusty bread.
I usually add a layer of breadcrumbs to au gratin to give a nice crust and some texture to the dish, however as this recipe of Julia Child's does not suggest breadcrumbs, I have not add the same. Similarly I have not added any herbs, though mint, tarragon or parsley would go well with the zucchini.