In a large skillet over medium heat, melt the margarine. Add the onion and celery and cook, stirring often, until tender, about 5 minutes. Add the zucchini, salt, thyme, red pepper flakes, and pepper. Cook until the zucchini is just tender, 8 to 10 minutes.
In a cup, dissolve the cornstarch in the half and half. Pour over the zucchini and cook, stirring, until thickened and coating the vegetables, about 2 minutes.
Remove from the heat and stir in the sour cream and 3 tablespoons of the cheese. Scrape into a 2 quart baking dish. Mix together the bread crumbs and remaining tablespoon of cheese. Sprinkle evenly over the top.
Bake until bubbly, about 20 minutes. Turn on the broiler and broil until the top is lightly browned, about 1 minute.