I love September for it's bounty of vegetables, warm days and the first touches of autumn in the light on on the trees. I still drink rose but can make a Zucchini Gratin on a cool day. Here's my go to recipe....
6 tablespoons butter, plus extra for topping
1 pound yellow onions, cut in 1/2 and sliced (3 large)
2 pounds zucchini, sliced 1/4-inch thick (4 zucchini)
2 teaspoons kosher salt
1 teaspoon freshly ground black pepper
1/4 teaspoon ground nutmeg
2 tablespoons all-purpose flour
1 cup hot milk
3/4 cup unseasoned bread crumbs
3/4 cup grated Gruyere
Preheat the oven to 400 degrees F.
Melt the butter in a large saute pan and cook the onions over low heat for 20 minutes, or until tender but not browned. Add the zucchini and cook, covered, for 10 minutes, or until tender. Add the salt, pepper, and nutmeg and cook uncovered for 5 more minutes. Stir in the flour. Add the hot milk and cook over low heat for a few minutes, until it makes a sauce. Pour the mixture into an 9 X 9 inch baking dish.
Combine the bread crumbs and Gruyere and sprinkle on top of the zucchini mixture. Dot with 1 tablespoon of butter cut into small bits and bake for 20 minutes, or until bubbly and browned.
I've also made this with cauliflower successfully. It freezes quite well if you don't mind the vegetables disintegrating a bit and I warm it straight from the freezer in a 350 degree oven for about 40 minutes. I serve it alongside most any meat dish, but I think it works really well with grilled italian sausages and a fresh green salad.