Ingredients
1 teaspoon ground cinnamon
2 ½ cups all-purpose flour, divided
1 ¾ cups granulated sugar, divided
⅓ cup cold unsalted butter, cubed
Baking spray with flour
1 cup sliced fresh strawberries
1 cup finely chopped fresh or thawed frozen rhubarb
½ cup cup unsalted butter, softened
1 large egg
2 teaspoons baking powder
1 teaspoon kosher salt
½ cup whole milk
Directions
Preheat oven to 350°F. Stir together cinnamon, ½ cup of the flour, and ¾ cup of the sugar in a medium bowl. Using your fingers or a fork, cut cold butter into sugar mixture until coarse crumbs form, leaving some larger pieces. Chill until ready to use.
Spray a 10-inch cast-iron skillet with baking spray. Stir together strawberries, rhubarb, and ¼ cup of the sugar in a medium bowl. Let sit, stirring occasionally, until macerated, about 10 minutes.
Meanwhile, beat softened butter and remaining ¾ cup sugar with a stand mixer fitted with a paddle attachment on medium-high speed until fluffy, about 4 minutes. Beat in egg until blended. Stir together baking powder, kosher salt, and remaining 2 cups flour in a medium bowl until combined. Add flour mixture to butter mixture alternately with milk, beginning and ending with flour mixture.
Drain liquid from macerated fruit, reserving liquid for another use. Gently fold fruit into batter. Spoon into prepared skillet, smoothing top with a spatula. Sprinkle evenly with sugar mixture.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 50 to 55 minutes. Cool 30 minutes before slicing. Serve warm, or cool completely.