Preheat oven to 350℉. Butter and flour a 9x13 baking pan shaking out any excess flour. Set aside.
Add 2 cups of the sifted cake flour and the brown sugar to a food processor. Pulse until mixed well. Add the pieces of cold butter and process until mixture becomes crumbly, about 30 seconds. Transfer mixture to a large bowl. Measure out 3/4 cup and reserve for the topping. Add the beaten egg to the remaining crumb mixture and mix with a rubber spatula.
In a small bowl, combine the remaining 1/2 cup sifted cake flour, baking powder, cinnamon, and lemon zest. Mix the flour mixture into the crumb mixture in 3 batches using a hand mixer alternating with the milk in two batches. Pour the batter into the prepared baking dish.
In a medium bowl, toss the reserved crumb mixture with the strawberries and rhubarb until they are evenly coated. Scatter the fruit and crumb mixture evenly over the batter. Bake until cake becomes golden brown and toothpick inserted into the center comes out clean, about 35-45 minutes. Let cool completely before serving. Top cake with a dusting of powdered sugar or a scoop of vanilla ice cream. YUM!
Notes
Lightly adapted from Blueberry Crumb Cake in Cooking at Home Cookbook by Chuck Williams.