Name: Spaghetti Squash Au Gratin Casserole
Prep time: 15 min
Cook time: 25 min
Total time: 40 min
Ingredients: 4 cups spaghetti squash strands, cooked (1 medium spaghetti squash); 1 teaspoon olive oil; 1 cup onion (1 small onion; diced); ½ cup sour cream (or Greek yogurt); 1 cup sharp cheddar cheese (shredded; divided); 2 eggs; ½ teaspoon garlic powder; ¼ teaspoon salt (or to taste)

Spaghetti squash au gratin is a creamy, cheesy casserole made with tender squash strands, sharp cheddar cheese, sour cream, and sautéed onions, then baked until golden and bubbly.

A serving of spaghetti squash au gratin being lifted out of a casserole dish.

Notes from my kitchen

This easy casserole is one of my favorite ways to dress up spaghetti squash without a heavy sauce or breadcrumbs.

It’s easier to make than you might think, too. Instead of wrestling with a raw spaghetti squash, I roast it whole and cut it open once it’s tender. That simple shortcut makes this a side dish I’m happy to make any night of the week.

Come holiday time, it’s a delicious lower-carb swap for starchier sides that carb-conscious and gluten-free friends can enjoy. And for something a little more fun, bake the mixture in a muffin pan so everyone gets their own spaghetti squash au gratin cup.

Marjory

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Notes from my kitchen
Ingredients
How to cook spaghetti squash for a casserole
How to make spaghetti squash au gratin casserole
📋 Recipe

Ingredients

Here is what you'll need to make this spaghetti squash au gratin recipe:

Spaghetti squash strands, eggs, cheddar cheese, sour cream, onion, olive oil, and garlic powder on a board.

Of special note:

Cooked spaghetti squash - The mild, slightly sweet strands pair especially well with sharp cheddar and creamy sour cream.
Sour cream adds creaminess and a slight tang. Plain Greek yogurt works well too and adds a little extra protein.
Eggs bind the spaghetti squash strands and creamy cheese mixture together so the casserole sets instead of becoming loose or watery. They also make it possible to bake the mixture in individual muffin cups.
Cheddar cheese - Sharp cheddar contrasts nicely with the mild spaghetti squash. For a different flavor, try Gruyère or Swiss. Shredding cheese from a block will give you the smoothest melt.

The recipe card below includes ingredient amounts, step-by-step instructions, and nutritional information.

How to cook spaghetti squash for a casserole

My preferred method is to cook the spaghetti squash whole in the oven.

Simply pierce the squash in several places and roast it whole at 425°F until tender, rolling it over halfway through. Let it cool for about 10 minutes, then cut it open, remove the seeds, and scrape the flesh into strands with a fork.

See my complete guide to cooking spaghetti squash whole for cooking times based on squash size, doneness tips, and the slow cooker method.

Spaghetti squash strands in a glass bowl in front of sheet pan and empty spaghetti squash gourds.

How to make spaghetti squash au gratin casserole

Once you have cooked spaghetti squash strands, this cheesy casserole comes together in four easy steps. The complete instructions are located in the recipe card.

Sauté the onion

Diced onions sautéing in a skillet for spaghetti squash au gratin.

Sauté the diced onion in olive oil until soft and translucent.

Mix the casserole

Spaghetti squash, sour cream, eggs, cheddar cheese, and sautéed onions mixed in a bowl.

Combine the cooked spaghetti squash, sautéed onion, sour cream, eggs, seasonings, and half of the cheddar cheese.

Assemble the casserole

Spread the mixture in the prepared baking dish and sprinkle the remaining cheddar cheese over the top.

Bake

Bake until the casserole is set, then broil briefly until the cheese on top is golden and bubbly.

Make spaghetti squash au gratin in a muffin pan

For individual servings, prepare the spaghetti squash mixture as directed, then divide it among greased muffin cups and top with the remaining cheddar cheese. Bake as directed, then let the cups rest for at least 10 minutes to set.

Run a small silicone or nylon spatula around the edges and gently lift each cup from the pan.

Spaghetti squash au gratin muffins stacked on a serving platter.

Love spaghetti squash?

Try creamy spaghetti squash alfredo, turn it into a spaghetti squash pizza crust, or warm up with spaghetti squash soup.

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📋 Recipe

Spaghetti Squash Au Gratin Casserole

Spaghetti squash au gratin is a creamy, cheesy side dish with sharp cheddar cheese, sour cream, and onions. Bake it as a casserole or in individual muffin cups.

5 from 2 votes

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Prep Time: 15

Cook Time: 25

Total Time: 40

Servings: servings

Author: Marjory Pilley

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Ingredients

▢ 4 cups spaghetti squash strands, cooked 1 medium spaghetti squash
▢ 1 teaspoon olive oil
▢ 1 cup onion 1 small onion; diced
▢ ½ cup sour cream or Greek yogurt
▢ 1 cup sharp cheddar cheese shredded; divided
▢ 2 eggs
▢ ½ teaspoon garlic powder
▢ ¼ teaspoon salt or to taste

Instructions

Preheat oven to 350°F.
Measure out 4 cups of spaghetti squash strands after draining off any excess liquid, and set aside.
Heat olive oil in a small skillet over medium heat. Add diced onion and saute for about 5 minutes or until tender.
Add cooked onion, spaghetti squash strands, sour cream, ½ cup of cheddar cheese, eggs, garlic powder, and salt to a large bowl and mix to combine.
Spread the mixture into an 8 x 10-inch casserole dish sprayed with nonstick cooking spray. For individual servings, divide the mixture among 18 greased muffin cups.
Sprinkle remaining cheddar cheese over the top of the spaghetti squash mixture.
Bake for about 25 minutes or until firm.
Turn the oven to broil and cook for 2-3 minutes more, or until the cheese is golden and bubbly.
Allow spaghetti squash au gratin to cool for 10 minutes before serving.

Notes

To cook spaghetti squash whole: Pierce the squash several times with a sharp knife and place it on a baking sheet.

Roast at 425°F for about 30 minutes, then check for doneness. Continue cooking and checking every 15 minutes until a knife easily pierces the skin and flesh.

Roll it over halfway through cooking for more even cooking. Continue to cook and check at 15-minute intervals until done. Avoid overcooking it since the strands will cook again in the casserole.

Let cool for about 10 minutes, then cut in half, remove the seeds, and scrape the flesh into strands with a fork.

Measuring cooked spaghetti squash strands: Spaghetti squash naturally holds moisture. Let the cooked strands sit briefly in a colander or drain off visible liquid before measuring the four cups. You don't need to wring the squash dry because the eggs help the casserole set.

Storage: Refrigerate leftovers in an airtight container for up to 3 days.

Make ahead: Cook the spaghetti squash up to 5 days ahead, assemble the casserole a day ahead and bake when ready, or bake the entire casserole and reheat the next day.

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Notes from my kitchen

Ingredients

How to cook spaghetti squash for a casserole