1cup[2 sticks or 227 g] unsalted butterat room temperature, sliced into 1 in [2.5 cm] pieces
2/3cup[130 g] granulated sugar
¼cup[28 g] cornstarch
16oz[454 g] rhubarbfresh or frozen, chopped into 1 in [2.5 cm] pieces 12 oz [368 g] raspberries, fresh or frozen
1teaspoonpure vanilla extract
For the crust
Adjust an oven rack to the middle of the oven. Preheat the oven to 350F [180C]. Grease a 9 x 13 in [22 by 33 cm] Lodge cast iron casserole pan and line it with a parchment sling.
In the bowl of a stand mixer fitted with a paddle, mix the flour, oats, granulated and brown sugars, baking soda, salt, on low speed to combine. Then add the butter and mix on medium-low speed until the mixture resembles coarse sand.
Press half of the flour mixture into the bottom of the prepared pan. Bake for 10 minutes. Prepare the filling while the crust is baking.
For the filling
In a small bowl, whisk together the granulated sugar, cornstarch, cinnamon, and salt.
In a large bowl, mix the rhubarb, berries, and vanilla together. Pour the sugar mixture over the fruit and stir gently with a spatula to evenly combine.
Remove the pan from the oven, spread the filling over the crust in an even layer, and sprinkle the remaining crust mixture evenly over the top. Bake for 50 to 60 minutes, until the crumbly top is light golden brown and the fruit juices have started to bubble. Transfer the pan to a wire rack to cool. Place the pan in the fridge and let the bars chill for 4 to 6 hours. Slice the bars and serve, serve with whipped cream if desired. Bars can be served cold or at room temperature and will keep in the fridge for up to 2 days.