Rhubarb Bars With Buttery Oat Crumble Topping

These rhubarb bars have a buttery oat crust, a soft jammy rhubarb filling and loose golden crumbs on top. One oat mixture makes both the base and the topping, so the recipe stays simple. The card lists 20 minutes of prep, about 55 minutes of baking and 1 hour 15 minutes in total, and it makes 12 bars in a 9×13-inch pan. Allow extra time for cooling before you slice them.

What Goes Into the Oat Crumble and Rhubarb Filling

Crust and crumble

• 2 cups old-fashioned oats
• 2 cups all-purpose flour
• 1 cup light brown sugar
• 1/2 cup granulated sugar
• 1 teaspoon baking powder
• 1/2 teaspoon cinnamon
• 1/2 teaspoon salt
• 1 cup unsalted butter, melted

Rhubarb filling

• 4 cups chopped fresh rhubarb
• 3/4 cup granulated sugar
• 2 tablespoons cornstarch
• 1 tablespoon lemon juice
• 1 teaspoon vanilla extract

Thinner rhubarb stalks are usually more tender and less stringy. The cornstarch thickens the juices so the bars slice cleanly without turning gummy, and the lemon juice keeps the fruit tasting fresh instead of only sweet.

For strawberry rhubarb bars, replace 1 cup of the rhubarb with chopped strawberries. If the berries are especially juicy, add an extra teaspoon of cornstarch to help the filling set.

How to Make Rhubarb Crumble Bars

1. Prep the pan. Preheat the oven to 350°F and line a 9×13-inch baking pan with parchment paper, leaving a little overhang on the sides so the bars lift out easily.
2. Make the crumble. In a large bowl, stir together the oats, flour, both sugars, baking powder, cinnamon and salt. Pour in the melted butter and stir until the mixture looks like damp sand with small crumbles.
3. Bake the crust. Press about two-thirds of the mixture firmly into the pan, compact and even with no dry patches. Bake for 12 minutes, just until the edges begin to turn lightly golden.
4. Mix the filling. While the crust bakes, combine the rhubarb, sugar, cornstarch, lemon juice and vanilla in another bowl. The rhubarb will start to release liquid, which helps make the jam-like filling.
5. Layer. Spread the filling over the warm crust, including the juices in the bowl. Sprinkle the remaining crumble over the top in loose clumps so some filling peeks through.
6. Bake again. Bake for 40 to 45 minutes, until the top is golden brown and the filling bubbles around the edges. The center may still look slightly soft and will firm up as the bars cool.
7. Cool before cutting. Let the bars cool completely. Chilling the pan for 30 minutes after cooling makes the cleanest cuts, especially with a thick filling layer.

Why Rhubarb Bars Turn Out Watery or Fall Apart

• Watery filling: The fruit released too much juice, or the bars were cut while still warm. Warm rhubarb filling can slide apart even when the crust is baked through. Bake until the edges bubble actively, allow several hours of cooling, and add an extra teaspoon of cornstarch if the rhubarb looks very juicy.
• Frozen rhubarb: It works, but thaw it first and drain off the extra liquid. It releases more moisture in the oven, so a little extra cornstarch helps.
• Top browning too fast: Tent the pan loosely with foil for the last 10 minutes.
• Crumble too fine: Squeeze some of the topping mixture into larger clusters before sprinkling for a crunchier top.

Serving, Storing and Freezing Rhubarb Bars

The bars are good chilled from the fridge or slightly warm with vanilla ice cream, and they travel well to potlucks because the crust stays sturdy once cooled. Cover the pan and refrigerate within 2 hours of cooling, and eat them within 3-4 days. To freeze, slice the cooled bars, put parchment between the layers in an airtight container and freeze for up to 2 months. Thaw them overnight in the refrigerator.

If you have a lot of rhubarb to use up, a strawberry rhubarb crisp uses a similar oat topping, and a strawberry rhubarb pie with a lattice crust is the classic option. For a brunch table, a rhubarb coffee cake with crumb topping goes well with these.

FAQ

Can I make rhubarb bars ahead of time?

Yes. They set as they cool, so they are easy to bake a day ahead. Keep them covered and refrigerated.

Can I make no-bake rhubarb bars?

Not with this recipe. Raw rhubarb stays firm and tart, so it needs baking. No-bake versions use a cooked rhubarb compote layered over a chilled oat crust, and the texture is softer.

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