Because of course ratatouille is one of those dishes that was clearly born of somebody trying to deal with an overabundant garden. (Like making pesto to use up all the basil.) If you survey a bunch of different recipes, as I am wont to do, you can easily see the bones of the ur-recipe, which is just to chop up all the veggies you have too much of and saute them together with whatever herbs you typically use for everything, et voila, it's dinner.
So that's what I did, except I didn't feel like standing at the stove forever, so I roasted them instead. At 350, in two batches of two baking sheets each, since I had so much. (4-6 of each of the main veggies, plus three small onions.) All indications are that it is going to be excellent. (It's in the fridge waiting to be dinner later, but everyone agrees it's much better the next day anyway.)
I'm not sure exactly how long I cooked everything; I think a key technique is to open the oven every five minutes to check progress and, more importantly, let all the steam out. That way the veggies actually start to brown. So just do that until they start to look done.