1. Cut the zucchini crosswise into one-inch slices, add the dill and garlic and boiling salted water to cover, and return to a boil. Reduce the heat, cover and simmer gently until the vegetable is tender; do not overcook. Drain, reserving two tablespoons of the cooking liquid. Discard the garlic.
2. Saute the mushrooms in butter five minutes, stirring occasionally. Stir in the flour and cook two minutes longer. Add the sour cream, zucchini and reserved cooking liquid, stirring constantly. Correct the seasonings and heat thoroughly but do not boil.
3. Transfer the mixture to a casserole and top with buttered bread crumbs. Brown quickly under high broiler heat.