Cheesy Zucchini Tomato Onion Casserole

This easy to prepare cheesy zucchini tomato onion casserole is a delicious way to use up all that fresh garden produce. It’s packed with nutrition, plenty of seasonal flavors and topped with cheesy goodness. Simply slice zucchini, tomatoes and sweet onions, season with garlic, toss in a casserole dish, sprinkle with two kinds of cheese and bake. That’s it! It’s the perfect side dish for any dinner. Oh, and don’t expect to have leftovers – it’s that good! Low carb, gluten-free and keto.

Cooked casserole on top of dark green napkin with tomato beside it.

Is there a boatload of zucchini coming out of your garden right now? Are you wondering what to do with it? Well, garden or no, zucchini is at the peak of its season and there’s no better time to make zucchini recipes than now. I’ve already made my death by chocolate zucchini bread, whipped some healthy bran zucchini muffins and baked a savory summer squash galette this season. The galette gave me the inspiration for this casserole. This is an easier preparation with a lot of the same flavors. 

Closeup of cheesy topping.

You probably know by now that zucchini is one of my favorite vegetables. There’s a good reason for that. Besides its great taste, it’s low in calories, has no saturated fat or cholesterol, is high in fiber and provides potassium, folates and Vitamins A & C. See, aren’t you lovin’ it now too? And it works great as a low carb, gluten-free substitute for pasta in almost any Italian dish like my tasty Zucchini Noodle Carbonara

Spoonful of cheesy zucchini and tomatoes.

But it doesn’t stop there! Zucchini, when used as a side dish, pairs well with almost any meal from chicken to pork to fish. So, come on, why not try this easy zucchini casserole for dinner tonight for some ooey gooey cheesy deliciousness. 

Frequently asked questions

Can I make this ahead of time?

You can prep it ahead of time but it is best baked and eaten warm, directly out of the oven.

Can I freeze this casserole?

I wouldn’t advise it. Zucchini is a watery vegetable, so freezing it will make the casserole soggy.

How long will it keep in the refrigerator?

It will easily keep up to 3 days. To reheat, place in the microwave until warmed.

Can I make this into a complete meal?

Absolutely! I sometimes add shredded or chopped up rotisserie chicken or cooked ground beef on top of the zucchini after it is baked and before sprinkling on the cheese.

Directions

Photo of ingredients on white table with labels.

Gather all the ingredients. Prepare the zucchini by cutting in half then slicing. Dice the tomatoes, slice the onion thinly and grate the cheeses.

Showing easy prep steps 1-6 for zucchini casserole.

  1. Place all the prepped vegetables in a large bowl along with chopped garlic. (photo 1)
  2. Season with Italian seasoning, salt & pepper. (photo 2)
  3. Add olive oil. (photo 3)
  4. Toss to coat. (photo 4)
  5. Pour into a greased  square baking dish. (photo 5)
  6. Cover with foil and bake for 30 minutes. (photo 6)

Finishing steps applying cheese after first bake.

  1. After the first bake, remove from oven and uncover. (photo 7)
  2. Sprinkle the cheese over the vegetables. This is a good place to vary it with different flavors of cheese. You can replace the cheddar with Gouda or even Gruyere. (photo 8)
  3. Make sure to cover well. (photo 9)
  4. Return to oven and bake uncovered for an additional 30 minutes. Serve immediately. (photo 10)

Overhead of cooked casserole with serving spoon in it on green napkin.

An easy, delightfully cheesy, side dish that is low carb, gluten free and keto friendly. Believe me, this cheesy zucchini casserole will soon become your family’s new favorite. Enjoy!

More delicious zucchini recipes

Blue Cheese Zucchini Beef Burgers – Yep, it’s a burger packed with plenty of nutritious veggies! It makes the hamburger extra moist and, believe me, no one will be able to tell you snuck some goodies in there.

Zucchini and Prosciutto Skewers – Skewered zucchini and prosciutto are grilled to perfection then served with a lemon basil pesto.

Bruschetta Chicken with Zoodles – makes a light, quick and easy weeknight dinner. Simply brown chicken in a large skillet, add freshly made bruschetta, toss in some zoodles and it’s ready!

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Cheesy Zucchini Tomato Onion Casserole

Get your zucchini on with this easy to prepare cheesy zucchini casserole. Slice up vegetables, season, grate some cheese and bake. That's it!
5 from 2 votes
Print Rate
Course: Side Dish
Cuisine: American
Prep Time: 20 minutes
Cook Time: 1 hour
Total Time: 1 hour 20 minutes
Servings: 4 servings
Calories: 261kcal
Author: Linda Warren

Ingredients

  • 2 zucchini, washed & halved lengthwise then cut into slices
  • 1 large tomato, cubed
  • 1 medium sweet onion, sliced thin, then halved
  • 6 garlic cloves, chopped
  • 1 teaspoon Italian seasoning
  • Salt & pepper to taste
  • 1 1/2 cup cheddar cheese, shredded
  • 1/2 cup Pecorino Romano or Parmesan, grated

Instructions

  • Preheat oven to 375 degrees. Lightly coat an 8x8-inch baking dish with olive oil.
  • Prepare vegetables by cutting zucchini in half lengthwise then slicing. Dice tomato and slice onion thinly.
  • In a large bowl, toss the vegetables together with Italian seasoning and salt and pepper to taste.
  • Place in prepared baking dish and drizzle a little more olive oil over vegetables. Cover with foil and bake for 30 minutes.
  • While zucchini is baking, grate both cheeses.
  • Remove casserole from oven, uncover, and sprinkle cheeses over top of vegetables.
  • Place back in oven and bake, uncovered, for another 30 minutes.

Notes

Frequently asked questions

Can I make this ahead of time?
You can prep it ahead of time but it is best baked and eaten warm, directly out of the oven.
Can I freeze this casserole?
I wouldn't advise it. Zucchini is a watery vegetable, so freezing it will make the casserole soggy.
How long will it keep in the refrigerator?
It will easily keep up to 3 days. To reheat, place in the microwave until warmed.
Can I make this into a complete meal?
Absolutely! I sometimes add shredded or chopped up rotisserie chicken or cooked ground beef on top of the zucchini after it is baked and before sprinkling on the cheese.

Nutrition

Calories: 261kcal | Carbohydrates: 10g | Protein: 17g | Fat: 18g | Saturated Fat: 11g | Cholesterol: 53mg | Sodium: 475mg | Potassium: 440mg | Fiber: 2g | Sugar: 5g | Vitamin A: 974IU | Vitamin C: 25mg | Calcium: 495mg | Iron: 1mg

This post has been updated with new photos and step-by-step directions. It was first published on June 7, 2015.

Cheesy Zucchini Tomato Onion Casserole
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