Rainbow Vegetable Gratin

Fresh squash and tomatoes are a couple of the many things we look forward to each summer. Former F&W culinary director at large Justin Chapple combines the two in his recipe for rainbow vegetable gratin. Yes, rainbow — because in addition to yellow squash, zucchini, and juicy heirloom tomatoes, the gratin is also packed with Yukon Gold potatoes, sweet potatoes (purple or orange), and red onion, too. The end result is six stunning layers that truly look like a rainbow, and would make an impressive addition to any meal.

Frequently asked questions

What's the difference between gratin and casserole?

A gratin is a casserole that's topped with cheese and/or breadcrumbs and finished under the broiler to produce a crusty, golden-brown exterior. Typically, vegetable gratins are carefully layered, as is this free-form version. Here, finely grated Parmigiano-Reggiano is combined with panko, chopped fresh thyme, kosher salt, and black pepper to form the breadcrumb mixture, which not only tops the gratin but adds texture and flavor between every layer of vegetables.

How are vegetables prepped for a gratin?

Each layer of this gratin is made with a different vegetable (yes, in this case, we consider tomatoes to be vegetables), and you'll need to peel and slice them accordingly before you start building. Dial in your knife skills or employ a mandoline to make even cuts; if you go with the latter, be careful and use a guard or a cut-resistant glove. Justin recommends using a serrated knife to slice the tomatoes and cutting the Yukon Gold potatoes last, in order to avoid them oxidizing. (And while you're at it, leave the skin on the Yukon Golds, too, so they hold their shape when baking.)

Notes from the Food & Wine Test Kitchen

For tender vegetables and a crisp, golden top, the gratin bakes in three stages. You'll initially bake it at 350°F for about an hour — you'll know it's ready for the next phase when you insert a paring knife and there's only a little bit of resistance. Then, remove the foil and sprinkle on the rest of those cheesy breadcrumbs. Rotate the pan front-to-back and return it to the oven for another 20 minutes (this time, the knife should come out easily when you test it). Leave the gratin in the oven and give it a quick, final blast under the broiler for two to three minutes. To make serving easy, allow the gratin to cool for 20 minutes before cutting.