01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease lightly with cooking spray.
02 - In a medium bowl, toss together the diced strawberries and rhubarb. Set aside while preparing the batter.
03 - In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until well combined.
04 - In a separate bowl, whisk the eggs, vegetable oil, whole milk, and vanilla extract until smooth and fully incorporated.
05 - Pour the wet ingredients into the dry ingredients. Mix gently with a spatula just until combined - do not overmix as this will result in tough muffins.
06 - Gently fold the strawberry and rhubarb mixture into the batter, taking care not to crush the fruit pieces.
07 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.
08 - In a small bowl, combine the flour, brown sugar, cinnamon, and salt. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
09 - Sprinkle the streusel mixture evenly over the tops of the filled muffin cups, pressing gently to adhere.
10 - Bake for 22 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
11 - Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.