Prep the oven and baking dish. Preheat your oven to 350°F. Grab a 9x9-inch baking dish and spray it lightly with cooking spray. Line it with parchment paper, letting the paper hang over the sides. This step is essential for easy removal later.
Make the fruit filling. In a medium mixing bowl, combine the diced strawberries and rhubarb. Sprinkle with sugar, lemon juice, and lemon zest. Gently toss everything together until the fruit is well coated and set aside to let the flavors meld.
Make the crumble mixture. In a large bowl, whisk together the flour, oats, baking powder, brown sugar, granulated sugar, and salt. Add in the butter and vanilla extract. Using a pastry cutter or your fingers, mix until the mixture is crumbly. Reserve about a cup of this mixture for the topping.
Assemble the bars. Press the remaining crumble mixture firmly into the bottom of the prepared pan. Evenly spread the fruit filling over the crumb layer, ensuring that it’s distributed well. Sprinkle the reserved crumble mixture over the top and gently press it down.
Bake. Transfer your baking dish to the preheated oven. Bake for 45 minutes to 1 hour, or until the top is golden brown and the filling is bubbly. Keep an eye on it towards the end to ensure it doesn’t burn.
Cool, cut, and serve. Once done, remove the dish from the oven and let it cool completely in the pan. Lift it out using the parchment paper, and cut into squares. Serve your delicious Strawberry Rhubarb Bars as is, or top with a scoop of vanilla ice cream for an extra treat.