3 tablespoons butter
1 tablespoon flour
1 cup milk
1 large sweet onion, cut in two from top to bottom and finely sliced
2 lbs small, firm zucchini or other summer squash, quartered lengthwise and cut into 1/2-inch lengths
Salt & freshly ground pepper
1/4 cup freshly grated Parmesan or Gruyere cheese
Handful of coarsely grated dry breadcrumbs
Optional: 1 Tablespoon dried thyme
Preheat oven to 400 degrees.
Over low hear, warm half the butter in a heavy casserole, add the onions, and sweat, covered, stirring occasionally with a wooden spoon, for about 30 minutes, or until melting, but uncolored. Add the zucchini and salt and cook, first covered, then uncovered, until tender but not falling apart.
Sprinkle the onion and zucchini with flour and stir gently. Raise the heat somewhat and add the milk, a few spoonfuls at a time, stirring until the sauce boils before adding more. Simmer gently for 10 minutes before seasoning with pepper, tasting for salt, and pouring the mixture into a buttered gratin dish.
Mix together the cheese and bread crumbs, sprinkle over the surface, and bake for 20 minutes, or until golden. Serve hot or tepid.