Preheat your oven to 180°C. Line the baking pan with baking paper carefully. Leave extra paper hanging over the sides. This helps remove the bars easily after baking.

In a large bowl, combine flour, oats, brown sugar, baking powder and salt. Stir everything together well. This mixture creates both the buttery crust and crunchy crumb topping.

Add cold butter cubes into the dry mixture. Use a pastry cutter or fork to mix. Continue until the texture looks crumbly like wet sand with small buttery pieces throughout.

Press half of the crumb mixture firmly into the prepared pan. Make an even layer across the bottom. This creates a sturdy crust that holds the fruity filling nicely.

In another bowl, combine strawberries, rhubarb, sugar, cornstarch, lemon juice and lemon zest. Stir gently until everything coats evenly. The mixture should look juicy and colorful before baking.

Spread the fruit mixture evenly across the crust layer. Make sure the fruit reaches all corners. This helps every bar get a delicious balance of sweet filling and crumb topping.

Sprinkle the remaining crumb mixture evenly over the fruit filling. Do not press it down too hard. Loose crumbs create a crisp and golden topping while baking beautifully.

Place the pan into the oven carefully. Bake for about 45 minutes until golden brown. The fruit filling should bubble slightly around the edges while the topping turns crisp.

Remove the bars from the oven and let them cool fully. Cooling helps the filling set properly. Cutting too early may cause the bars to fall apart easily.

Lift the bars from the pan using the baking paper edges. Cut into squares carefully using a sharp knife. Serve plain or with whipped cream for extra sweetness.