Traditional ratatouille, the stew, starts with a base of tomatoes, garlic, onions, spices and seasonings with layers of vegetables e.g. eggplant, bell peppers, cooked in the base, then finished off either in the oven or over the stove-top.

What you are seeing here today may not be traditional ratatouille but to capture the spirit of French chef and restaurateur - Joël Robuchon of The Complete Robuchon: "The secret of a good ratatouille is to cook the vegetables separately so each will taste truly of itself".
This is how the roasted vegetables in this recipe are done. Seasoned and cooked in separate trays within a single oven roast, so that when they are combined in a single dish, each vegetable will taste truly of itself.

Similarly, the tomatoes are also "cooked" separately in a "no-cook" way - immersed in extra virgin olive oil, garlic, and basil, salt and black pepper.
Ratatouille is usually served as a side dish, but may also be served as a meal on its own (accompanied by pasta, rice or bread).