To prepare the ratatouille, just throw ingredients in a pot and simmer until tender.
Dust the Roughy with flour and sauté until crispy, then top with the ratatouille mixture.
You can cook the fish with the ratatouille or cook it separately. All at once is easier and quicker. A small dice is better--it cooks faster; and if the tomatoes are finely diced, you really don't need to peel them.