Keto Tuna Casserole

Ingredients

This is a delicious, filling, well-seasoned mixture of canned tuna, chopped red onion, mayonnaise, and cheese, baked until golden and bubbly.

Here's an overview of the ingredients - the exact measurements are listed in the recipe card below:

Canned tuna: I use skipjack tuna in this recipe.

Mayonnaise: I like using avocado oil mayonnaise.

Dijon mustard: It's creamier and less vinegary than yellow mustard. I do recommend using it here rather than using yellow mustard.

Shredded Gruyere cheese: Other melty cheeses such as cheddar or provolone should work too. But Gruyere is especially flavorful.

Chopped red onion: Make sure it's finely chopped.

To season: Salt, black pepper, and cayenne pepper. The cayenne pepper doesn't make the casserole very spicy, at least to my palate, but it does add a nice subtle kick.

Chopped parsley: I use it mostly for garnish, so you can skip it if you don't have it on hand. It does add a nice splash of color, though.

Variations

You can use tuna in olive oil instead of tuna in water. It's delicious!
Try using sour cream instead of mayonnaise. Both work, but personally, I prefer mayonnaise.
Dill makes a nice addition to this recipe.
Sharp cheddar or provolone are good substitutes for Gruyere. Smoked Gouda is also very good.

Instructions

Making this tuna casserole is so easy! Scroll down to the recipe card for detailed instructions. Here are the basic steps:

Simply mix all the ingredients together, reserving some of the cheese and the parsley. (Photos 1-2).
Next, transfer the mixture to a small baking dish and sprinkle the top with the remaining cheese. (Photo 3).
Bake the casserole until heated through, about 15 minutes in a 400°F oven. Garnish with parsley and serve. (Photos 4-6).

A six-photo collage showing the steps for making a tuna casserole.

Recipe FAQs

Why is my casserole dry?

This casserole can become dry if the ratio of tuna to creamy ingredients (mayonnaise and mustard) is too high.

I find that ⅓ cup of mayo works well, but depending on the brand of tuna you use, you might need to increase it and use up to ½ cup.

Can you make a tuna casserole without noodles?

Yes! I do it all the time. It's a different recipe than classic ones, and you'll want to serve it with something for scooping it out, such as fresh-cut veggies, keto crackers, or pork rinds. But it's very, very good.

Can you freeze this casserole?

Yes. Once completely cool, you can wrap the baking dish in plastic wrap followed by a layer of foil. Freeze it for up to three months. Thaw it in the fridge.

Reader Review

On Instagram, @shelikeshergreens posted this gorgeous photo of her casserole. She says, "I made my own mayonnaise for this casserole. I didn’t have gruyere so I used cheddar instead. I added dill to the mix to bring some freshness to it. This was amazing with the sautéed beans!"

A reader photo of a keto tuna casserole with a side of green beans.

Storing Leftovers

The leftovers keep well in the fridge for 3-4 days, or you can freeze them for up to three months if you want to keep them for longer. Gently reheat them in a 300°F oven or in the microwave at 50% power.