Prepare the streusel topping by combining flour, granulated sugar, brown sugar, and salt in a food processor. Add cold butter and pulse until coarse crumbs form. Mix in walnuts and set aside.

Preheat your oven to 425°F (220°C) and line a muffin tin with paper liners.

Hull and dice strawberries and chop rhubarb, then toss together in a bowl and set aside.

In a medium bowl, whisk together flour, baking powder, and salt.

In a large bowl, beat together butter and granulated sugar until fluffy. Add eggs, milk, and vanilla extract, mixing until combined.

Fold the dry ingredients into the wet mixture until just combined, then gently fold in the strawberries, rhubarb, and walnuts.

Spoon the batter into muffin tin, filling each cup about two-thirds full. Sprinkle streusel topping on each.

Bake for 15-18 minutes or until golden brown and a toothpick comes out clean. Cool in the tin for 10 minutes before transferring to a wire rack.