Summer Vegetable Gratins with Intense Flavor

Look for a golden crust and caramelized vegetable juices on the sides of the pan—two signs of a perfectly baked gratin. Here, it's zucchini and summer squash.

Summer's here, and like many cooks, I'll soon be faced with the "what to do with all the excess garden vegetables" challenge. I can't say that I'm crazy about zucchini bread, and I can't keep feeding the extra tomatoes to my dog (he gobbles them up like candy). Fortunately, I've found a delicious solution that uses large amounts of these vegetables: a summer vegetable gratin—layers of fresh tomatoes and other produce like eggplant, zucchini, or squash, with a bit of cheese, a generous amount of fresh herbs, a drizzle of olive oil, and a crunchy breadcrumb topping, all melted together by slow roasting.

As the gratin cooks, the vegetables shrink, releasing their moisture and concentrating their flavor. The finished dish is a hearty blend of flavors that can stand on its own as dinner with some crusty bread, or that can be the perfect side dish for grilled meats. Leftovers, serendipitously, are better than the first day's meal.

Five steps to perfect summer gratins

Vegetable gratins aren't hard to make; they just take a little prep time and a little layering handiwork to fit them in the pan. In fact, the only way you can really ruin one of these is by undercooking it. The longer the gratin is in the oven, the more its flavors develop.

First, you need the right dish. I'd like to say that this is the excuse you needed to buy a beautiful earthenware tian made in the South of France. (A tian is the French name for an ovenproof earthenware dish, used to cook all kinds of gratins.) But I won't, because these recipes will taste just as good in a 7x11-inch Pyrex dish. An oval dish looks pretty, but any heavy, shallow, 2-quart, ovenproof dish will work.

Next, choose the freshest vegetables and herbs. In summer, this shouldn't be hard, but I've found that when I make gratins with zucchinis and squash from the grocery store (cold-stored for who knows how long) and with those winter tomatoes harvested millions of miles away, even these long-cooked gratins suffer in flavor. This doesn't stop me from making them in winter, but they taste best in summer.

To take advantage of the best vegetables and herbs you can find, don't feel constrained by the ingredient lists in the recipes here. Once you've followed one or two of the recipes to learn the method, take a look at the chart "Customizing your gratin" below for inspiration to create your own gratin. This is a great way to use all those funny round and twisted squash or tiny eggplant from the farmers' market, or even freshly harvested baby potatoes. At the start of the summer, you can use the first green tomatoes. To complement your produce, make sure that your other ingredients, including the olive oil and cheese, are of the best quality. Their flavors will play a dominant role in the finished dish.

Sharpen your knife and prep your vegetables. Most of these gratins start with a layer of caramelized onions or leeks, so slice those first and start sautéing them slowly while you prepare your other ingredients. Partially cook potatoes and eggplant before using them in the gratins; otherwise, they never seem to get fully cooked in the gratin, probably because of their low moisture content. I roast eggplant slices in the oven (see Eggplant & Tomato Gratin), and I parboil potatoes (see Red Potato Gratin).

Next, slice your zucchini and squash on the bias into nice thin ovals (if they're particularly thick, cut them in half first), discarding the ends. Try for even 1/4-inch slices. Fatter pieces won't cook as evenly or layer as tightly, but at the same time, don't go paper-thin or the vegetables will melt away. Be sure to toss raw zucchini and squash with olive oil (a coating of fat helps conduct heat around the vegetables and will ensure they get fully cooked). Use a serrated knife to cut the tomatoes into slices that are also quite thin, but not so thin that they fall apart. Put the tomatoes on a shallow plate to let some of their juices drain. Trim and mince the herbs and then arrange all the prepped ingredients in small bowls and shallow plates around your oiled baking dish.

Cut the vegetables evenly, on the bias, so your gratin looks good and cooks evenly.

Let the sliced tomatoes sit for a bit and then drain off the juices.

Add full flavor with a first layer of caramelized onions.

With all the prepped vegetables around you, begin assembling the gratin. Spread the sautéed onions or leeks in one thin layer in the dish. Then, starting at one (narrow) end of the dish, arrange a row of vegetable slices, slightly overlapping one another, across the width of the dish. Prop up the row at a 60-degree angle to the dish. Sprinkle generously with cheese (and anything else the recipe calls for) and lay down a row of the next vegetable, overlapping the first by at least two-thirds. As you work your way along the dish, push the rows back towards the end of the dish where you started. By compacting the vegetables this way, you should get most of what you've sliced into the pan, but by the very nature of this hand-crafting, you'll almost always wind up with a few odd leftover slices. Don't forget to top the finished gratin with a drizzle of olive oil, a good covering of breadcrumbs, and extra cheese.

Cook the gratin until it's well-browned and greatly reduced in volume. These gratins usually cook perfectly in about an hour and ten minutes in a 375°F oven. But after testing them in four different home kitchens, I have to say that cooking time and oven temperature aren't the best way to determine doneness: looking at them is. (Every home oven I used was either "slow" or "fast" compared to my own—who knows which was correctly calibrated?—and each gratin cooked in a different amount of time.)

In any case, it's wise to get into the habit of learning how to judge doneness without depending on cooking times, and these gratins give you some good visual clues. After several minutes in the oven, the gratins begin to bubble as the vegetables release their moisture. The bubbling becomes quite vigorous and, as the vegetable juices reduce, the bubbling lessens. When the gratins are close to being done, the bubbles are just visible around the edges of the pan. (You can also tilt the pan to see how much juice collects at the end.) The top of the gratin will be well-browned, and the mound of vegetables will have shrunk and pulled away from the sides of the pan.

Let the whole dish rest for at least 15 minutes before serving. If there are still a lot of juices in the pan, you can spoon servings out with a slotted spoon. But don't discard the juices. Leave the leftovers in the juices overnight; then reheat them the next day. The wonderful caramelized flavor from the roasted juices will be even better. In fact, you can even fully cook these gratins several hours ahead, let them cool, and reheat them again before serving.