Step 1: Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan or spray with baking spray. Line with parchment paper if desired.
Step 2: Prepare crumb topping: In a medium bowl, combine the ½ cup all-purpose flour, ¼ cup granulated sugar, ¼ cup packed light brown sugar, and ¼ teaspoon ground cinnamon. Add the ¼ cup cold, cubed unsalted butter. Use a pastry blender or your fingertips to cut the butter into the dry ingredients until coarse crumbs form. Set aside.
Step 3: Prepare fruit: In a separate medium bowl, combine the sliced strawberries and chopped rhubarb with ¼ cup of the granulated sugar. Toss gently to coat and set aside. For best results, lightly toss the fruit with 1 tablespoon of the 2 ¾ cups flour before adding to the batter.
Step 4: Cream butter and sugar: In a large mixing bowl, using an electric mixer, cream together the 1 cup softened unsalted butter and the remaining 1 ¾ cups granulated sugar until light and fluffy (about 3-5 minutes).
Step 5: Add eggs and vanilla: Beat in the 3 large eggs one at a time, mixing well after each addition. Stir in the 1 teaspoon vanilla extract.
Step 6: Combine dry ingredients: In another medium bowl, whisk together the remaining 2 ¾ cups minus 1 tablespoon all-purpose flour (from Step 3 if you tossed fruit in flour), 2 teaspoons baking powder, ½ teaspoon baking soda, and ¼ teaspoon salt.
Step 7: Alternate dry and wet ingredients: Gradually add the dry ingredients to the wet ingredients, alternating with the 1 ½ cups buttermilk, beginning and ending with the dry ingredients. Mix on low speed until just combined; do not overmix.
Step 8: Fold in fruit: Gently fold the sugared strawberries and rhubarb into the cake batter until just evenly distributed.
Step 9: Assemble and bake: Pour the batter into the prepared 9x13 inch baking pan, spreading it evenly. Sprinkle the reserved crumb topping generously over the batter.
Step 10: Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. If the crumb topping starts to brown too quickly, you can loosely tent the cake with aluminum foil.
Step 11: Cool and serve: Let the cake cool in the pan on a wire rack for at least 30 minutes before slicing and serving. This allows the cake to set and flavors to meld.