Uniform Slices: Be sure to slice your vegetables uniformly to all be approximately the same size and thickness so that everything cooks evenly.
Preheat oven to 350°F | 180°C. Spray a deep-dish pie plate with cooking spray and set aside.
Heat a cast iron or heavy skillet over medium-high heat. When a water droplet sizzles on the surface, add 2 teaspoons of olive oil and swirl to coat skillet.Carefully place whole onion slices on the skillet. Allow the onion slices to brown and crisp on one side for a few minutes, and then carefully turn them over using a spatula and brown the other side.Allow onion slices to cool, and then separate onion rings from one another and set aside.
Make an herb mixture by combining the dried oregano, thyme, rosemary, salt, pepper, and chili powder in a small bowl. Set aside.
In a medium bowl, combine crushed or diced tomatoes, remaining olive oil (2 teaspoons), vinegar, garlic, fresh basil, and all but 1 teaspoon of the herb mixture.
Pour tomato mixture evenly into the bottom of a deep dish pie plate. (We use a cast iron pie dish.)
Arrange sliced veggies around the edge of the plate in a pattern that makes you happy. (I did tomato-zucchini-eggplant-zucchini-eggplant-repeat.)
Put the pan-fried onion mixture in the middle of the veggie-ring. Arrange more veggies in the middle, until you are either out of veggies or space, whichever comes first.
OPTIONAL: At this point, you can spray the whole thing with a little cooking spray or brush it with just a bit of olive oil to encourage browning if you would like. Doing so also seems to make the whole thing look better after it is done roasting.
Sprinkle remaining dried herb mixture over the top of the prepared ratatouille.
Bake in preheated oven for an hour, or until the sauce is bubbly and the veggies are tender (but not mushy!!)
Garnish with fresh basil and freshly grated Parmesan or Romano.
Alternate (aka "non-fussy") Prep: Put the crushed tomato mixture in your baking dish. Top with pan-fried onions. Throw the veggies on top. Bake as directed.