1. Make the Fruit Filling:

In a large saucepan, combine the chopped rhubarb, mashed strawberries, and lemon juice.

Cover and cook over medium heat for 5 minutes, allowing the fruit to start softening and releasing juices.

In a small bowl, combine the 1 cup sugar and the cornstarch. Stir this mixture into the saucepan with the fruit.

Bring the mixture to a boil, stirring constantly.

Cook and stir for 2 minutes more, or until the filling has thickened noticeably.

Remove the saucepan from the heat and set the filling aside.

2. Prepare Oven and Combine Dry Cake Ingredients:

Preheat oven to 350°F (175°C). Grease a 13×9-inch baking dish.

In a large bowl, combine the 3 cups all-purpose flour, 1 cup sugar, baking powder, baking soda, and salt. Whisk them together.

3. Cut in Butter for Cake Batter:

Cut the 1 cup of cold butter pieces into the dry ingredients using a pastry blender or two knives until the mixture resembles coarse crumbs.

4. Add Wet Cake Ingredients:

In a separate bowl, beat the buttermilk, room temperature eggs, and vanilla extract together.

Stir the wet ingredients into the crumb mixture just until combined. Do not overmix; the batter will be somewhat thick.

5. Layer the Cake:

Spread half of the batter evenly into the bottom of the greased 13×9-inch baking dish.

Carefully spread the reserved strawberry-rhubarb filling evenly over the bottom layer of batter.

Drop the remaining batter by tablespoonfuls over the fruit filling. It doesn’t need to cover completely; it will spread during baking.

6. Make and Add the Topping:

For the topping, melt the ¼ cup of butter in a small saucepan over low heat (or in the microwave).

Remove from heat. Stir in the ¾ cup all-purpose flour and ¾ cup sugar until the mixture resembles coarse crumbs.

Sprinkle the crumble topping evenly over the batter.

7. Prepare for Baking & Bake:

Lay a sheet of aluminum foil on the lower oven rack to catch any potential juice spillovers during baking.

Place the coffee cake on the middle oven rack.

Bake for 40-45 minutes, or until the topping is golden brown and a toothpick inserted into the cake portion (avoiding the filling) near the center comes out clean.

8. Cool and Serve:

Cool the coffee cake completely on a wire rack before cutting and serving. This allows the filling to fully set.

If desired, serve slices warm or at room temperature with whipped cream and additional orange zest. Enjoy!