Let’s bake this delicious Strawberry Rhubarb Coffee Cake layer by layer! First, prepare the flavorful fruit filling.

Make the Filling: In a large saucepan, combine the 3 cups sliced rhubarb, 1 quart (about 4 cups) mashed fresh strawberries, and 2 tablespoons lemon juice. Cover the pan and cook over medium heat for about 5 minutes, allowing the fruit to soften and release some juices.

In a small bowl, whisk together the 1 cup sugar and 1/3 cup cornstarch until combined (this prevents lumps). Stir this sugar-cornstarch mixture into the fruit in the saucepan. Bring the mixture to a boil, stirring constantly. Once boiling, continue to cook and stir for 2 minutes, or until the filling has noticeably thickened. Remove the saucepan from the heat and set the filling aside to cool slightly while you make the batter.

Make the Cake Batter: Preheat your oven to 350°F (175°C). Grease your 13×9-inch baking dish thoroughly. In a large bowl, whisk together the 3 cups all-purpose flour, 1 cup sugar, 1 teaspoon baking powder, 1 teaspoon baking soda, and 1/2 teaspoon salt.

Add the 1 cup of cold butter pieces to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs.

In a separate medium bowl, beat together the 1 1/2 cups buttermilk, 2 room temperature eggs, and 1 teaspoon vanilla extract. Pour this wet mixture into the crumb mixture. Stir gently with a fork or spatula just until the dry ingredients are moistened. Do not overmix; the batter might be slightly lumpy.

Assemble the Cake: Spread half of the cake batter evenly into the bottom of the prepared 13×9-inch baking dish. Carefully spoon the slightly cooled strawberry-rhubarb filling over this batter layer, spreading it gently to cover.

Drop the remaining cake batter by rounded tablespoonfuls evenly over the fruit filling. It doesn’t need to cover completely; gaps are fine and allow the filling to peek through.

Make the Topping: For the simple crumble topping, melt the 1/4 cup butter in a small saucepan over low heat (or in the microwave). Remove from heat. Stir in the 3/4 cup all-purpose flour and 3/4 cup sugar until the mixture resembles coarse crumbs. Sprinkle this topping evenly over the dolloped batter.

Bake the Cake: Place a sheet of aluminum foil on the lower oven rack to catch any potential drips from the bubbling fruit filling. Place the coffee cake pan on the middle oven rack. Bake for 40 to 45 minutes, or until the topping is golden brown and a wooden toothpick inserted near the center comes out clean.

Carefully remove the baked coffee cake from the oven and place the pan on a wire rack. Let the cake cool completely, or at least significantly, before slicing and serving. This allows the filling to set further. Enjoy!