Summer Vegetable Gratin

Abby Knowles is the owner of the Boulder Kitchen Collective. She serves as a chef and operational food service consultant for events, workshops and private clients. Knowles focuses heavily on food systems in Colorado. She works directly with farmers and ranchers throughout the region to find the best and most sustainably harvested products. She sees regenerative agriculture as our future. Being able to work with food from start to finish—and doing this in a way that heals our soil and planet—is her true passion.

Knowles says she created this recipe because “it includes so many wonderful tastes of summer! You may have all you need right in your very own garden or can easily pick things up at your local farm stand. The rest of the ingredients are minimal and most likely on hand”—like cheese and oil.

Summer Vegetable Gratin

Serves: 6-8

2 tablespoons ghee (or vegetable oil)
2 yellow onion, chopped
1 yellow squash, sliced
1 green zucchini, sliced
1 small narrow eggplant, sliced
5–6 plum tomatoes (Roma or the like), sliced
1 tablespoon fresh thyme, pulled from stems
¾ cup extra-virgin olive oil 
¼ cup Parmigiano-Reggiano, freshly grated
2 tablespoons fresh basil, chiffonade 
Kosher salt and freshly ground pepper, to taste

Preheat oven to 350°F.

Heat the ghee over medium heat. Add chopped onions and sauté, stirring often, until translucent (about 20 minutes).

Slice the squash, zucchini and eggplant approximately ¼ inch thick (a mandoline works well). Place in a large bowl, drizzle with a portion of your olive oil (about ½ cup) and season with salt and pepper. Carefully toss to coat.

Slice the tomatoes using a serrated knife, drizzle with the remaining ¼ cup of olive oil and season with salt and pepper.

Spread onions evenly in a 12-inch round baking dish, cast-iron skillet or any oven-safe dish of a similar size.

Layer the eggplant on top of the onions, slightly overlapping each slice, until the entire dish is covered. Repeat this method with the zucchini and then the squash. Reserve any olive oil the remaining in the veggie bowl.

Grate the Parmigiano-Reggiano over the top of the squash in an even layer and sprinkle the pulled thyme over the cheese.

Layer the sliced tomatoes as you did the other vegetables. Place these carefully as this will be the top of the dish when it’s presented at the table.

Place on the middle rack of oven for 1 hour. The tomatoes should slightly brown but not burn or completely dry up. If they’re not slightly browned, bake for another 15–30 minutes. Your veggies will be done when your they are completely tender. To test, there should be no resistance when pierced with a knife.

Remove from oven. Add the final layer of cheese and drizzle with the remaining oil. Place back into the oven for roughly 5 minutes to melt the cheese.

Garnish just before serving with the fresh chiffonade of basil.

Recipe courtesy of

Abby Knowles is the owner of the Boulder Kitchen Collective. She serves as a chef and operational food service consultant for events, workshops and private clients. Knowles focuses heavily on food systems in Colorado. She works directly with farmers and ranchers throughout the region to find the best and most sustainably harvested products. She sees regenerative agriculture as our future. Being able to work with food from start to finish—and doing this in a way that heals our soil and planet—is her true passion.

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