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Tomato, Zucchini, Squash and Caramelized Onion Gratin

Tomato, Zucchini, Squash and Caramelized Onion GratinOur friends invited us for dinner and I was asked to bring a side dish. I was craving something warm and comforting since it had been raining all day. I had a zucchini and yellow squash to use up so I decided to try a gratin. Gratins normally have a rich white sauce that accompanies the vegetable but I wanted to keep it a bit healthy so I opted to use Parmesan cheese instead. I wanted it to be creamy so I threw in a layer of caramelized onions and garlic. It was beautiful, full of great veggies and flavors, easy to make, and totally delicious! It went really well with my friend Kyle’s Turkey Moussaka (which was amazing and I can’t wait to re-create it). We had a lovely evening with friends, drank great wine, watched our kids play together, and ate a spectacular dinner. A good night indeed.

Tomato, Zucchini, Squash, and Caramelized Onion Gratin:


  • 6-7 large Roma tomatoes, sliced
  • 1 large zucchini (or 2 small ones), sliced
  • 1 large yellow squash (or 2 small ones), sliced
  • 1 tsp olive oil
  • 1 onion, sliced
  • 1 clove of garlic, sliced very thin
  • 1/4-1/2 cup of Parmesan cheese, grated
  • 1/2 tsp dried basil
  • 2-3 leaves of basil, chopped for garnish

Tomato, Zucchini, Squash and Caramelized Onion Gratin

How to Make a Tomato, Zucchini, Squash, and Caramelized Onion Gratin

Preheat the oven to 375 degrees. Coat a large and deep baking dish (I used a large corning ware casserole dish) with olive oil cooking spray.

Slice the zucchini, yellow squash, onion, and tomatoes into thick slices.

Heat the olive oil in a skillet over medium heat. Once the pan is hot add the onion slices and sauté for 15-20 minutes or until golden brown and caramelized- make sure you season with sea salt to taste. Add the garlic to the onions and sauté, stirring frequently for 60 seconds then remove from heat and let cool.

Layer the dish with first tomatoes then yellow squash, a bit of Parmesan then the layer of caramelized onions and garlic, then the zucchini, and finally the last layer of tomatoes. I seasoned each layer (except the onions) with a little bit of sea salt and pepper. Top with the remaining Parmesan cheese and then sprinkle the dried basil on top and cover with a lid or tin foil.

Tomato, Zucchini, Squash and Caramelized Onion Gratin

Bake for 20-25 minutes. Remove the lid or tin foil and broil for 3-4 minutes or until golden brown and crispy on top. Let cool for a few minutes. Garnish with fresh basil. Enjoy.


Click here for a printable version of this recipe

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  1. This looks wonderful. A perfect side dish, or a main course if you ask me. I love that you didn’t use a cream sauce.

  2. I love your food Pam! I want to have a potluck with you! This looks great and is so my style, healthy but delicious!

  3. All of the veggies look amazing Pam. Even your kids enjoy vegetables? You know you’re a great cook when that happens 🙂

  4. OK, THIS is speaking to me. It’s going on the list of morsels to make. I make a zucchini squash casserole but never did it with caramelized onions. Thank you!!!!!!

  5. Right up my alley. I’ve made a similar dish – this one looks great. I’m going to put it in my recipe file!

  6. Looks gorgeous, I have a thing with sliced tomatoes, I try to avoid them for no reason, your gratin makes me wanna try it all over again. Still… do you think I can replace tomatoes with something else? 😉

  7. Imagine your life without evenings like these…pretty grim?

    I love this recipe…except for the cheese, it is healthy and definitely worth drooling over!

  8. Not having tried any of this it seems so alien, but would relish being able to try the taste.

    The pictures are lovely.

  9. This is like a TIAN that I make.
    I love this dish!
    Fall is so nice because squash is in full gear!
    zucchini is so versatile. I love it!

  10. Evenings with friends or family are my favorite!

    This gratin looks like the best thing to dive a fork into! It has all of my favorite! Keep cooking!

  11. I love how your gratin looks!!!! That crispy golden top crust really complements the lovely colors of the dish, doesn’t it!

  12. These photos are terrific! I love squash and cheese and tomatoes and basil and onions …etc! This looks so pretty, and I bet your friends loved it! How fun to get together with the entire family and share a meal! Smiles all around, I’m sure!

  13. Since my tomatoes, onions and squash are all making at the same time, I can definitely put this recipe to good use! Thanks for sharing.. 🙂

  14. Just made a half batch of this tonight for myself and the BF- very good! He’s GF/CF so I had to use vegan parm substitute, but it still turned out lovely. Will make again!

  15. I plan to make this veggie casserole this coming Saturday evening for a pot-luck dinner we’ve been invited to. Will let you know the reaction of guests, including hubby and mine on Sunday so keep posted and thanks..it looks delicious!

  16. cannot read your recipes when printed. Big Disappointment. Type is so light it is not readable. Done on purpose?

    1. K. Bbreidenbach,

      I’m sorry you are having a hard time reading the printed recipe. I just printed it and it was not too light for my husband, children, and me to read. I’m wondering if the toner in your printer may be running low?

      I hope you enjoyed the recipe.