Cheddar Baked Zucchini Tomato Onion Casserole

Here’s my go-to zucchini tomato onion casserole recipe, with layers of fresh summer vegetables, herbs from the garden, and a light sprinkle of cheese that melts perfectly into every bite.

This casserole has become my family’s favorite way to use up our garden vegetables during the summer months. I often make an extra batch to share with neighbors, and it’s amazing how such simple ingredients can create such a comforting meal.

What Kind of Zucchini Should I Use?

For this casserole, medium-sized zucchini (about 6-8 inches long) are your best bet since they’re tender but still hold their shape when cooked. Try to pick zucchini that feel firm and heavy for their size, with smooth, shiny dark green skin. Smaller zucchini tend to be sweeter and have fewer seeds, while really large ones can be watery and have tough seeds that might make your casserole soggy. If you’re shopping at the farmers market in summer, you might also find yellow zucchini (also called summer squash) – these work just as well and can add nice color to your dish. Just be sure to avoid any zucchini with soft spots or wrinkled skin, as these are signs they’re past their prime.

Options for Substitutions

This easy casserole is pretty flexible with substitutions. Here’s what you can swap:

• Zucchini: You can swap zucchini with yellow summer squash, or use a mix of both. If you’re feeling adventurous, try eggplant slices – just salt them first and let them sit for 30 minutes to remove excess moisture.
• Tomatoes: Any type of fresh tomato works here. Roma, beefsteak, or even cherry tomatoes (halved) are good options. In a pinch, you can use drained canned diced tomatoes.
• Sweet onion: Regular yellow or red onions work just fine. Red onions will add a nice pop of color to the dish.
• Italian seasoning: No Italian seasoning? Mix dried basil, oregano, and thyme in equal parts. Fresh herbs work great too – just double the amount.
• Cheddar cheese: Feel free to use Monterey Jack, Gruyere, or Mozzarella instead. Each will give a slightly different but equally good flavor.
• Pecorino Romano/Parmesan: These hard cheeses can be swapped for each other, or try Asiago or aged Manchego for something different. The key is using a hard, salty cheese for that punch of flavor.

Watch Out for These Mistakes While Cooking

The biggest challenge when making zucchini casserole is dealing with excess moisture – zucchini releases a lot of water during cooking, which can make your casserole soupy instead of perfectly layered. To prevent this, try salting your sliced zucchini and letting it sit for 15-20 minutes, then pat it dry with paper towels before assembling the casserole. Another common mistake is overcrowding the baking dish, which leads to steaming instead of proper roasting – use a large enough dish so vegetables can be arranged in a single layer, allowing them to caramelize nicely. For the best cheese topping, wait until the last 10 minutes of baking to add your cheese mixture, as adding it too early can result in an overly browned or burnt top. Finally, let the casserole rest for 5-10 minutes after baking – this helps the cheese set properly and makes it easier to serve clean portions.

What to Serve With Zucchini Tomato Casserole?

This cheesy veggie casserole works great as a side dish, but you can also make it the star of the show! For a complete meal, try serving it alongside some grilled chicken breast or baked fish – the Italian seasonings in the casserole pair really nicely with both. If you’re keeping things vegetarian, serve it with a scoop of quinoa or brown rice to soak up all those tasty juices. I also like to add a slice of crusty bread on the side because, let’s be honest, you’ll want something to scrape up every bit of that melted cheese from your plate!

Storage Instructions

Keep Fresh: This tasty casserole will stay good in the fridge for 3-4 days when stored in an airtight container. The vegetables might release a bit more liquid as they sit, but that’s totally normal and doesn’t affect the taste at all!

Freeze: While you can freeze this casserole, I wouldn’t really recommend it since the zucchini and tomatoes can get pretty watery when thawed. If you do freeze it, use it within 2 months and know the texture might be a bit different.

Reheat: To warm up leftovers, pop them in the microwave for 1-2 minutes, or heat in the oven at 350°F for about 10-15 minutes until heated through. If you notice extra liquid, you can drain it off before serving. A quick sprinkle of fresh cheese on top while reheating helps bring back that nice melty texture!

Preparation Time	15-20 minutes
Cooking Time	60 minutes
Total Time	75-80 minutes
Level of Difficulty	Easy

Estimated Nutrition

Estimated nutrition for the whole recipe (without optional ingredients):

• Calories: 600-700
• Protein: 35-45 g
• Fat: 40-50 g
• Carbohydrates: 20-30 g

Ingredients

• 2 zucchini, sliced after halving lengthwise
• 1 large tomato, diced
• 1 medium sweet onion, thinly halved slices
• 6 garlic cloves, chopped
• 1 tsp italian herb blend
• Salt and black pepper, as needed
• 1 1/2 cups shredded cheddar cheese
• 1/2 cup grated pecorino romano or parmesan

Step 1: Preheat Oven and Prepare Baking Dish

Preheat your oven to 375°F (190°C).

Lightly coat an 8×8-inch baking dish with a little olive oil to prevent the vegetables from sticking during baking.

Step 2: Prepare and Season the Vegetables

• 2 zucchini, sliced after halving lengthwise
• 1 large tomato, diced
• 1 medium sweet onion, thinly halved slices
• 6 garlic cloves, chopped
• 1 tsp Italian herb blend
• salt and black pepper, as needed

Cut the zucchini in half lengthwise and then slice into half-moons.

Dice the tomato and thinly slice the onion into half-moons.

Chop the garlic.

In a large bowl, combine the sliced zucchini, diced tomato, sliced onion, and chopped garlic.

Add the Italian herb blend, and season with salt and black pepper to taste.

Toss everything together well so the vegetables are evenly coated with the seasoning.

Step 3: Bake the Vegetables

Transfer the seasoned vegetables to the prepared baking dish, spreading them out evenly.

Drizzle a little more olive oil over the vegetables to help them roast nicely.

Cover the dish with foil and bake in the preheated oven for 30 minutes.

Step 4: Grate the Cheeses

• 1 1/2 cups shredded cheddar cheese
• 1/2 cup grated Pecorino Romano or Parmesan

While the vegetables are baking, grate the cheddar cheese and the Pecorino Romano or Parmesan cheese.

Prepare them so they’re ready to sprinkle over the casserole.

I like to use freshly grated cheese for the best texture and flavor.

Step 5: Top with Cheese and Finish Baking

• shredded cheddar cheese (from Step 4)
• grated Pecorino Romano or Parmesan (from Step 4)

Remove the baking dish from the oven and carefully take off the foil.

Evenly sprinkle the grated cheddar cheese and grated Pecorino Romano (or Parmesan) over the baked vegetables.

Return the dish to the oven, uncovered, and bake for another 30 minutes or until the cheese is bubbling and lightly golden.