Preheat your oven to 350°F and line a 9x13 inch baking dish with parchment paper for easy removal. While the oven heats, prepare your fruit: hull and slice the strawberries into 1/4-inch thick rounds, then trim and cut the rhubarb into 1/2-inch pieces. Having everything prepped before you start mixing ensures a smooth, efficient assembly process.

In a large bowl, whisk together the flour, baking soda, baking powder, and salt. Stir in the oats, brown sugar, and granulated sugar until evenly combined. Melt the butter and mix it with the vanilla extract, then pour this mixture into the dry ingredients and stir until the mixture resembles wet sand with visible oat pieces throughout. I like using quality butter like Kerrygold here because it really enhances the rich, buttery flavor of the base.

Press two-thirds of the oat mixture firmly into the prepared baking dish, creating an even bottom layer that will support the fruit filling. Set the remaining one-third of the oat mixture aside for topping. In a separate bowl, toss the prepared strawberries and rhubarb with the lemon juice to coat evenly.

In a small bowl, whisk together the sugar and cornstarch until no lumps remain. Toss this mixture with the lemon-coated fruit until evenly coated—the cornstarch will thicken the fruit juices as the bars bake, preventing a soggy bottom layer. Spread the fruit mixture evenly over the crust, then crumble the reserved oat mixture over the top in an even layer, pressing down gently so it holds together.

Bake in the preheated 350°F oven for 45-50 minutes, until the top is golden brown and the fruit filling is bubbling slightly at the edges. The bars are done when the crust is set and firm to the touch. I always let the bars cool completely in the pan before cutting—this gives the cornstarch time to fully set and keeps everything from crumbling apart.

Allow the bars to cool completely in the baking dish at room temperature for at least 2 hours (or until fully cooled). Once cooled, use the parchment paper to lift the entire slab out of the dish, then cut into squares. Serve as is or with whipped cream if desired.