Using a sharp knife, a mandoline or another slicing device, slice the green and yellow zucchini into paper-thin rounds.
Do the same with the red onion and the potatoes. With a serrated knife, cut the tomatoes into thin slices.
In a small bowl, combine garlic, thyme and parsley; set aside.
In the baking dish, arrange vegetables in tight, overlapping layers in the following order: green zucchini, yellow zucchini, red onion, potato and tomato. Repeat with remaining vegetables.
Season with salt and pepper, and add some of the herb/garlic mixture between the layers. Drizzle vegetables generously with olive oil. Bake for 60 to 75 minutes, or until vegetables are tender when pierced with a fork. Serve warm with grilled meat or fish.