Combine the flour, sugar, brown sugar, and salt in a food processor and pulse briefly. Add the cold cubed butter and pulse until the mixture resembles coarse breadcrumbs with some pea-sized pieces remaining—this texture is key for a crumbly, delicious topping. Add the walnuts and pulse just until combined. Transfer to a small bowl and set aside. I always use cold butter cubed right from the fridge because it creates those desirable larger crumbs that don't disappear into the batter during baking.

Preheat your oven to 425°F and line a muffin tin with paper liners. While the oven heats, hull and dice the strawberries into roughly 1/2-inch pieces and chop the rhubarb into similar-sized pieces. Toss the fruit together in a small bowl and set aside—keeping them separate until the last minute prevents excess moisture from accumulating and making the batter soggy.

In a medium bowl, whisk together the flour, baking powder, and salt. This combines your leavening agents evenly throughout and helps the muffins rise uniformly and have an even crumb structure.

In a large bowl, beat together the softened butter and sugar until light and fluffy, about 2-3 minutes—this incorporates air and creates a tender crumb in the final muffins. Add the room-temperature eggs one at a time, beating well after each addition. Pour in the milk and vanilla extract and stir until just combined. The batter should be smooth and homogeneous at this point.

Pour the dry ingredient mixture from Step 3 into the wet mixture from Step 4 and fold gently with a rubber spatula until just combined—do not overmix, as this can create tough muffins. Once the dry ingredients are barely incorporated, fold in the chopped walnuts and then gently fold in the fruit from Step 2. I like to add the fruit last and fold gently so the pieces stay intact and distribute evenly throughout rather than sinking to the bottom.

Divide the batter evenly among the lined muffin cups, filling each about two-thirds full. Sprinkle approximately 1 tablespoon of the streusel topping from Step 1 over each muffin, pressing gently so it adheres to the batter without sinking in.

Bake at 425°F for 15-18 minutes, until a toothpick inserted into the center of a muffin comes out with just a few moist crumbs (not wet batter). The tops should be golden brown and the streusel should be toasted. Allow the muffins to cool in the tin for 10 minutes, then transfer them to a wire rack to cool completely. This resting time in the tin allows the muffins to set slightly so they don't fall apart when you remove them.