Let’s bake these delicious Gluten-Free Raspberry Rhubarb Crumble Bars:

1. Make the Raspberry Rhubarb Compote:

• In a heavy-bottom saucepan, combine the diced rhubarb, raspberries (fresh or frozen), cane sugar, and the zest and juice of one lemon.
• Place the saucepan over medium heat. Bring the mixture to a hard boil, stirring occasionally.
• Once boiling, reduce the heat slightly to maintain a steady simmer. Let it simmer for 10 minutes, stirring occasionally, until the fruit has broken down.
• In a very small bowl, make a slurry by whisking the cornstarch or tapioca starch with a tablespoon or two of water or juice from the pot until smooth. Stir this slurry into the simmering fruit mixture.
• Continue to cook for 3 minutes longer, stirring, until the mixture has visibly thickened.
• Remove the saucepan from the heat and stir in the vanilla extract. Let the compote cool for at least 15 minutes (it can cool while you prepare the crust).

2. Preheat Oven and Prepare Crumble Mixture:

• While the compote cools, preheat your oven to 350°F (175°C).
• Line an 8×8-inch baking pan with parchment paper, leaving an overhang on two sides to act as “handles” for easy removal later.
• In a large mixing bowl, whisk together the oat flour, old-fashioned oats, brown sugar, tapioca starch, baking powder, sea salt, and cinnamon until well combined.
• Pour in the melted shortening or butter. Stir with a spatula or spoon until the mixture is well combined, evenly moistened, and forms a crumbly dough. No white streaks of flour should remain.

3. Pre-bake the Crust:

• Take a heaping cup of the prepared oat mixture and firmly press it into the bottom of your parchment-lined 8×8 baking pan. Use the back of a spoon or a flat-bottomed measuring cup to press it together tightly and create a smooth, even crust.
• Prick the crust all over a few times with a fork. This helps prevent it from puffing up during baking.
• Bake the crust for 12 minutes, until it is fragrant and appears lightly browned around the edges.
• Remove the pan from the oven and let the pre-baked crust cool for about 10 minutes. Keep the oven on.

4. Assemble and Bake the Bars:

• Spread 1 ¼ cups of the cooled raspberry rhubarb compote evenly over the top of the pre-baked crust.
• Take the rest of the oat crumble mixture and sprinkle it evenly over the compote layer to form the topping.
• Place the baking pan back in the oven.
• Bake for an additional 25-30 minutes. The bars are done when the fruit filling is bubbling around the edges and the crumble topping is golden brown.

5. Cool Completely Before Cutting (Crucial Step!):

• Remove the pan from the oven and set it aside on a wire rack to cool completely.
• Once fully cooled, refrigerate the bars for at least 1-2 hours. This chilling step is essential for the filling to set completely, ensuring you can cut clean, sturdy bars.

6. Slice and Serve:

• Once chilled and firm, use the parchment paper overhang to lift the entire block of crumble bars out of the pan and onto a cutting board.
• Using a large, sharp knife, cut into squares or rectangles. Serve chilled or at room temperature.

Patience during cooling and chilling is key for perfect bars!