Strawberry Rhubarb Crumb Cake

Combine three-fifths cup sugar and cornstarch in a substantial saucepan; stir vigorously the rhubarb and strawberries. Heat gently over medium heat; cook until it thickens, approximately 2 minutes. Take the saucepan off the heat, stir in lemon juice, and allow it to cool down.

While the filling is cooling off, preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Lightly grease a 9x13-inch baking dish.

Combine three cups flour, one cup sugar, baking powder, and baking soda in a large mixing container. Use two knives or a pastry blender to cut in one cup cold butter until the mixture resembles coarse fragments.

Whisk buttermilk, eggs, and vanilla together in a separate mixing bowl; stir into the flour mixture until it's just moistened. Spoon two-thirds of batter into the prepared pan; spread the cooled filling over the top, then cover with the remaining one-third of batter.

Combine three-quarters cup sugar and half a cup flour in a bowl. Use two knives or a pastry blender to cut in one-quarter cup cold butter until the mixture resembles coarse fragments; sprinkle over batter.

Bake in the preheated oven until the topping is golden brown and a tester inserted into the center comes out clean, 45 to 50 minutes. Let it cool on a wire rack.