Preheat your oven to 350°F and grease a 9x13 inch baking dish (or similar size). While the oven heats, cut the rhubarb into 1/2-inch pieces and quarter the strawberries, keeping them separate. Measure out all dry ingredients for both the crumb topping and cake batter into separate bowls so everything is ready to go. This upfront organization prevents mistakes and keeps the baking process smooth.

In a medium saucepan over medium heat, combine the rhubarb, strawberries, 1/2 cup sugar, cornstarch, water, and lemon zest. Stir frequently and cook for about 5 minutes until the mixture becomes syrupy and the fruit begins to soften but still holds its shape. The cornstarch will thicken the filling and prevent it from being watery in the finished cake. Set the filling aside to cool slightly while you prepare the batter.

In a small bowl, combine the brown sugar, 1/2 cup flour, and cinnamon. Add the 6 tbsp cold butter (cubed into 1/4-inch pieces) and use your fingertips or a fork to work the mixture until it resembles coarse breadcrumbs with pea-sized clumps throughout. I like to use cold butter straight from the fridge for the best texture—the small pieces create those desirable crumbly pockets. Set this mixture aside until you're ready to top the cake.

In a large bowl, beat the 3/4 cup softened butter and 1 1/2 cups sugar together using an electric mixer on medium speed for 3-4 minutes until the mixture is light, fluffy, and pale in color. This creaming process incorporates air into the batter, which helps the cake rise and creates a tender crumb. Scrape down the bowl occasionally to ensure everything is evenly combined.

Add the 4 room-temperature eggs one at a time to the creamed butter-sugar mixture, beating on medium speed after each addition until fully incorporated before adding the next. Pour in the vanilla and mix until combined. Room-temperature eggs blend more smoothly into the batter, creating a better emulsion and resulting in a more tender cake. Scrape the bowl as needed to ensure no streaks remain.

In a separate small bowl, whisk together the 3 cups flour, baking powder, baking soda, and salt. Reduce your mixer speed to low and alternately add the dry mixture and sour cream to the batter, starting and ending with the dry mixture (add dry, then sour cream, then dry, then sour cream, then dry). Mix just until combined after each addition—overmixing at this point can make the cake tough. The sour cream adds moisture and tanginess that balances the tartness of the rhubarb and strawberries.

Spread half of the batter from Step 6 evenly into the prepared baking dish. Pour the cooled strawberry-rhubarb filling from Step 2 over the batter, spreading it in an even layer. Dollop the remaining batter over the filling and gently spread it to cover (it doesn't need to be perfectly even). Sprinkle the crumb topping from Step 3 evenly over the batter. Bake at 350°F for 50-60 minutes until a toothpick inserted into the cake portion (avoiding the fruit) comes out clean or with just a few moist crumbs. The top should be golden brown. Cool the cake completely in the pan on a wire rack before slicing and serving.