Summer Vegetable Gratin

This recipe takes time but its layers of squash, caramelized onions, tomatoes, and Parmesan- bread crumb topping pack in a lot of umami. It is also very pretty and therefore great for guests, including a buffet or potluck. Pairs well with grilled meats or fish.
Based on a recipe by Rebecca Hays, Cook Illustrated, July & August 2008, p 13.

Prep Time1 hour 30 minutes
Cook Time45 minutes
Total Time2 hours 15 minutes
Course: Side Dishes - Vegetables, Vegetarian
Servings: 8 servings
Author: Catherine Christiano


  • 1 tablespoon olive oil


  • 1 pound zucchini ends trimmed and sliced crosswise into 1/4 inch thick slices
  • 1 pound yellow summer squash ends trimmed and sliced crosswise into 1/4 inch thick slices.
  • 1 teaspoon salt


  • 1 1/2 pounds ripe tomatoes field tomatoes are best, 3 to 4 large, sliced 1/4 inch thick
  • 1/2 teaspoon salt

Caramelized Onions

  • 1 tablespoon olive oil
  • 2 medium onions about 3 cups, halved lengthwise and sliced thin pole to pole
  • 1/4 teaspoon salt
  • 1/4 teaspoon ground black pepper

Oil - Garlic Mixture

  • 3 tablespoons olive oil
  • 2 medium garlic cloves about 2 teaspoons, minced
  • 1/2 teaspoon ground black pepper
  • 1 tablespoon minced fresh thyme leaves

Bread Crumb Topping

  • 1 large slice white sandwich bread torn into quarters
  • 2 ounces grated Parmesan cheese about 1 cup
  • 2 medium shallots about 1/4 cup, minced
  • 1 tablespoon olive oil


  • 1/4 cup chopped fresh basil leaves


  • Toss zucchini and summer squash slices with 1 teaspoon salt in large bowl; transfer to colander set over bowl. Let stand until zucchini and squash release at least 3 tablespoons of liquid, about 45 minutes. Rinse zucchini to reduce salt. Arrange slices on triple layer paper towels; cover with another triple layer paper towels. Firmly press each slice to remove as much liquid as possible.
  • Place tomato slices in single layer on double layer paper towels and sprinkle evenly with 1/2 teaspoon salt; let stand 30 minutes. Place second double layer paper towels on top of tomatoes and press firmly to dry tomatoes.
  • Adjust oven rack to upper-middle position and preheat to 400°F. Brush 13 x 9- inch baking dish with 1 tablespoon oil; set aside.
  • Meanwhile, heat 1 tablespoon oil in 12-inch non-stick skillet over medium heat until shimmering. Add onions, remaining 1/2 teaspoon salt, and 1/4 teaspoon pepper; cook, stirring occasionally, until onions are softened and dark golden brown, 20 to 25 minutes. Set onions aside.
  • Combine garlic, 3 tablespoons oil, remaining 1/2 teaspoon pepper, and thyme in small bowl. In large bowl, toss zucchini and summer squash in half of oil mixture, then arrange in greased baking dish. Arrange carmelized onions in even layer over squash. Slightly overlap tomato slices in single layer on top of onions. Spoon remaining garlic-oil mixture evenly over tomatoes. Bake until vegetables are tender and tomatoes start to brown on edges, 40 to 45 minutes.
  • Meanwhile, process bread in food processor until finely ground, about 10 seconds. (You should have about 1 cup crumbs.) Combine bread crumbs, remaining tablespoon oil, Parmesan, and shallots in medium bowl. Remove baking dish from oven and increase heat to 450 degrees. Sprinkle bread-crumb mixture evenly on top of tomatoes. Bake gratin until bubbling and cheese is lightly browned, 5 to 10 minutes. Sprinkle with basil and let sit at room temperature 10 minutes before serving.


The first steps in this recipe are to help reduce the amount of liquid generated by the squash and tomatoes - don't skip. Even following the directions results in extra liquid at the bottom of the baking dish. It might help to try and drain more liquid out of the zucchini and tomatoes by salting earlier and allowing to sit longer.
To make this recipe earlier in the day you intend to serve it, follow all instructions up to the point the baking dish is placed in the oven.
If serving as part of a meal for guests, pair with simply prepared meat or fish; ie. roasted or grilled. Steamed corn or roasted potatoes also pair well.
Copyright 2023, Catherine Christiano. Website by Catherine Christiano.