Summer peppers, eggplants, tomatoes and zucchini are upon us. I know that these can all be layered in a quick ratatouille that is both beautiful and efficient, but you know what? That has NEVER worked for me.
These are finicky vegetables that tend towards soggy grossness. I prefer to create a firm roasted barrier on each vegetable. And thus I must cook them all separately. I actually think this is not that laborious, it just means that you need to take them out when each vegetable is properly roasted (so you need to keep checking every ten minutes).
But it also means that if you chop the eggplant first, you can pop it in the 425 oven when it is ready to go (coated in olive oil) and then work on your next veggie.
Then when they are all roasted, I toss them together and their flavors meld just fine.