Preheat your oven to 350°F (175°C). Line a 9x9 inch baking pan with parchment paper. Leave some paper hanging over the sides. This helps you lift the bars out easily later.

In a large bowl, whisk the flour and oats together. Add the brown sugar, baking powder, and salt. Stir until there are no large sugar lumps. This is the base for your crust.

Add the cold, cubed butter to the flour mixture. Use a pastry cutter to mix it in. Work the butter until it looks like coarse crumbs. The butter bits should be pea-sized.

Take half of the crumb mixture and put it in the pan. Press it down firmly with your fingers. Make sure it covers the entire bottom. This creates a solid base for the fruit.

In a medium bowl, combine strawberries and rhubarb. Add the granulated sugar, cornstarch, lemon juice, and zest. Toss everything gently until the fruit is well coated. The cornstarch thickens the juice.

Pour the fruit mixture over the pressed bottom crust. Spread it out into an even layer using a spatula. Make sure the fruit reaches all the corners of the pan.

Take the remaining crumb mixture and sprinkle it over the fruit. Do not press it down this time. Leave it loose so it stays crunchy. Ensure most of the fruit is covered.

Place the pan in the center of the oven. Bake for 40 to 45 minutes. The top should be golden brown and the fruit bubbly. The smell will be absolutely amazing!

Remove the pan from the oven. Let it cool completely on a wire rack. This is the hardest part! Cooling allows the bars to set so they don't fall apart.