Blueberry Oatmeal Bars

By Leslie Kiszka · Apr 14, 2019 · Updated Apr 2, 2023

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These blueberry oatmeal bars are just the right amount of sweet with their juicy blueberry filling, soft and buttery oatmeal crust, and golden crumb topping! Contains no eggs, and can easily be made gluten free and dairy free. Keep the blueberry train rolling with my blueberry muffin bread or blueberry smoothie bowl.

Golden oatmeal topping on slices of purple blueberry bars on a blush colored plate

Christine said: “Great recipe! I doubled the batch. I’m going to freeze the remainder and use as crumbles on ice cream or in yogurt. ⭐⭐⭐⭐⭐“

Deep thought on this Sunday: What if they had called blueberries “purpleberries”?

Every time I mash or cook a blueberry and I see the purple inside, that's what I think. It's almost like I resent them for being called blueberries. Because it's almost like false advertising. Sure, they're blue on the outside, but aren't we taught that it's what's on the inside that counts?

What if this recipe were actually called Purpleberry Oatmeal Bars? It sounds like something straight out of Willy Wonka. But I guess we have plenty of oddly named fruits: dragon fruit, kumquats, boysenberries.

And breadfruit – IT'S NOT BREAD, PEOPLE. FFS.

I should really put in charge of naming things, I'm really good at it. Examples:

Dragon fruit = Pink Poppy Seed Alien Football
Kumquat = Oblong Oranges
Boysenberries = Almost Raspberries
Breadfruit = Goosebump Limes

Impressed? Thought so.

Square white baking dish filled with oatmeal mixture next to a bowl of blueberries and lemon zest

Ingredients for blueberry oatmeal bars

1 cup old-fashioned rolled oats, can substitute gluten free oats
¾ cup all purpose flour, can substitute 1:1 gluten free flour
¼ cup light brown sugar
¼ teaspoon kosher salt
6 tablespoons unsalted butter, melted (can substitute coconut oil, melted)
½ teaspoon pure vanilla extract
¼ cup granulated sugar
1 tablespoon freshly squeezed lemon juice, about half a lemon
Zest of one lemon
1 teaspoon cornstarch
12 ounces fresh or frozen blueberries*

How to make blueberry oatmeal squares

Preheat oven to 375° and line an 8×8 baking dish with parchment paper and coat with nonstick spray. Set aside.
In a large bowl, combine oats, flour, brown sugar and salt.
Add melted butter and vanilla extract, and stir until flour is completely combined and mixture is lumpy.
Set aside a little less than 1/2 of the mixture in a small bowl, and press the rest into the bottom of your prepared pan. Set aside.
In a medium bowl, combine granulated sugar, lemon juice, lemon zest, and cornstarch.
Add blueberries and carefully stir and/or toss the mixture to coat the blueberries.
Pour blueberry mixture evenly over crust in prepared baking dish.
Top with reserved oatmeal mixture – you'll still have some of the fruit showing through, and that's just fine!
Bake for 38-42 minutes or until the top is golden brown.
Place on a wire rack to cool completely (or place in the fridge to cool it more quickly). Use the edges of the parchment paper to lift the bars out of the pan, slice, and serve.

Square white baking dish filled with blueberry oatmeal mixture

Square white baking dish filled with golden blueberry oatmeal mixture that was freshly baked

What if it's not blueberry season?

Depending on where you live, you might be looking at this recipe thinking to yourself, “Leslie, it is not yet the appropriate time for me to go blueberry picking and now you have made me sad”.

Fear not! I usually make these with frozen blueberries and they are equally as delicious. So just grab a bag out of your store's frozen section and you're all set.

If you can make this recipe as is, fabulous! Easy peasy. But if you have dietary restrictions, or maybe your pantry is just lacking in the right ingredients, there are some substitutes you can make without it getting complicated.

Ingredient substitutions

Don't have fresh blueberries? Just grab some frozen ones! All you'll need to do is thaw them, rinse them and drain them before using.
Gluten free? Use gluten free oats and gluten free flour instead. I like to use Bob's Red Mill Gluten Free Organic Old Fashioned Oats (affiliate link) and Bob's Red Mill Gluten Free 1 to 1 Baking Flour (affiliate link).
Dairy free? Swap out the melted butter for some melted coconut oil – same amount.

Trial and error lessons

The base:
- I wanted the base to be chewy, buttery and sturdy enough to hold the filling. One time I used too much butter and it spread too much and never cooked through.
- Another time I tried a 1:1 ratio of oatmeal and flour, and the crust was too thick and seemed dry.
Sugar content: I had tried to avoid adding sugar, but it needed it for the right flavor. And the 1/2 teaspoon of pure vanilla extract is what really brought it all together for me.
Overcomplication of steps: I had also tried making the crust and crumb topping as two separate things, with two different sets of ingredients. It was entirely too complicated, and didn't need to be. Using the same thing for both works out perfectly.
Fresh vs. canned: I also wanted the filling to be freshly made and not some canned pie filling – but I struggled with getting the right mix of ingredients for a long time. An embarrassingly long time.
Adding lemon: I knew I wanted lemon juice and zest in there, but the amount of liquid that the lemon juice added really screwed me up. Blueberries get super juicy when they cook, and I just couldn't get the ratios right.
- Solution? Cornstarch: The key to my eventual success was the right amount of cornstarch. Too much and it was weirdly gelatinous. Too little, and the filling was too wet and the bars just fell apart. One teaspoon was just right.

Golden oatmeal topping on slices of purple blueberry bars

Can I make them in a 9″x13″ pan?

I haven't tried to make these in a 9″x13″ dish, but I would imagine if you doubled the recipe you'll just end up with slightly thicker bars as a result.

How many blueberry bars does this make?

This recipe (made in an 8″x8″ pan) can yield 9 large bars, or 16 smaller bars. Another way of saying that is, “You can cut these into 16 bars, or 9 if you want to be able say you only ate one and get more bang for your buck.”

I also have not tried making these with a mix of berries, but I'm aiming to soon! I'm thinking blueberries and blackberries, with a little extra lemon zest.

How to store blueberry oatmeal squares

Store in the fridge, covered for up to 3 days. They'll become softer as time goes on, so eating them within the first day is recommended.

Reheat by microwaving for 10-15 seconds.