Traditionally this fish would be cooked in parchment paper, and opened at the table (See NY Times article: "The Envelope Please: Cooking En Papillote") but I find the task somewhat tedious and prefer the much easier-to-use aluminum foil.
Haddock or Cod work best in this recipe and the few simple ingredients make it easy to throw together – especially in the summer when the zucchini is abundant and the tomatoes are at peak flavor.
You can make the fish packets ahead and just bake them when you’re ready for dinner.