Preheat oven to 400 degrees.
In a small saucepan or skillet, combine the olive oil, garlic, fresh oregano and thyme. Heat over low heat until it comes to a gentle simmer. Turn off the heat and let it steep while prepping the vegetables.
Slice the potatoes, zucchini, eggplant, and tomatoes 3/8-inch thick. Cut the onion in half and slice approximately 3/8-inch thick.
Combine the potatoes, zucchini, and eggplant in a large bowl. Pour half the infused oil over the vegetables including the garlic, add 1/4 teaspoon dried oregano, 3/4 teaspoon salt and black pepper to taste, and toss until coated.
Pour the petite diced tomatoes into the bottom of a large baking dish, such as a 13 x 9-inch or large oval gratin dish. Season with remaining salt, freshly ground black pepper and remaining dried oregano.
Place the potatoes, zucchini, and eggplant in the dish over the tomatoes, alternating and overlapping until they're all used. If any are left over, tuck them evenly throughout the baking dish. Ensure the garlic slices are evenly distributed in the baking dish.
Tuck the tomato slices in among the other vegetables so they're evenly distributed in the baking dish. Do the same with the red onion slices. (It should have a rustic appearance.)
Drizzle the remaining infused oil over the vegetables as well as any from the bowl that held the potatoes, zucchini, and eggplant.
Cover loosely with aluminum foil. Bake for 30 minutes. Remove the foil and continue baking until the potatoes are tender and the vegetables are lightly browned, approximately 45 minutes.
Let stand 15 minutes. Serve with crusty bread.