Split onions into rings. Lay half of them (one onion) with three cloves of minced garlic on the bottom of an 11 by 7 inch baking pan.
Slice zucchini halves lengthwise into 1/4 inch pieces, cutting to, but not through small ends. Layer zucchini and tomato slices over onions, spreading zucchini into fans. Top with the rest of the onions and garlic.
Pour broth or wine in a pot and bring to a simmer. Spoon warmed broth over vegetables and coat with cooking spray. Season with salt, pepper, basil, oregano, marjoram, and thyme.