Prep the pan. Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang on two sides for easy lifting. Lightly grease the parchment.

Make the fruit filling. In a saucepan, combine chopped rhubarb, strawberries, 3/4 cup granulated sugar, lemon juice, and vanilla. Cook over medium heat, stirring, until the fruit releases juice and starts to soften, about 5–7 minutes.

Thicken the mixture. Stir the cornstarch with 2 tablespoons of water to make a slurry. Pour it into the fruit and cook, stirring, until thick and glossy, 1–2 minutes. Remove from heat and set aside to cool slightly.

Mix the crust and crumble. In a large bowl, whisk flour, oats (if using), salt, cinnamon (if using), 1/3 cup granulated sugar, and 1/3 cup brown sugar. Pour in the melted butter and stir until the mixture looks like damp sand and clumps when pressed.

Form the base. Transfer about 2/3 of the crumb mixture to the pan. Press it firmly and evenly into the bottom to form a solid crust. Use the bottom of a measuring cup to pack it down.

Add the filling. Spread the strawberry rhubarb mixture evenly over the crust. Leave a tiny border around the edges to prevent overflow.

Top with crumble. Sprinkle the remaining crumb mixture over the fruit, leaving some gaps. If you like extra crunch, finish with a light sprinkle of turbinado sugar.

Bake. Bake for 35–42 minutes, until the top is golden and the fruit is bubbling around the edges. The center should look set, not soupy.

Cool completely. Let the pan cool on a rack to room temperature, then chill for at least 1 hour. This helps the bars slice cleanly.

Slice and serve. Lift out using the parchment overhang. Cut into squares or bars. Dust with powdered sugar if you like, and serve as is or with whipped cream or ice cream.