In a food processor, combine the flour, sugar, brown sugar, and salt, then pulse until the mixture resembles coarse sand. Add the cold butter cubes and pulse until the mixture forms pea-sized crumbs—this creates a tender, crumbly texture that won't be dense. Pulse in the finely chopped walnuts until just combined, then transfer to a small bowl and set aside. The cold butter is key here; it prevents the topping from becoming too compact and gives you those desirable rustic crumbles.

While the streusel sets aside, hull and dice the strawberries into 1/2-inch pieces and slice the rhubarb into 1/2-inch thick pieces—this uniform sizing ensures even cooking throughout the muffins. Preheat your oven to 425°F and line your muffin pan with paper liners. I find that prepping the fruit now, while the oven heats, keeps the workflow smooth and prevents any fruit from oxidizing or breaking down.

In a medium bowl, whisk together the flour, baking powder, and salt. This step aerates the dry ingredients and distributes the leavening evenly, which helps the muffins rise uniformly and achieve a tender crumb.

In a separate bowl, whisk together the melted butter and sugar until combined, then add the eggs one at a time, whisking well after each addition to create an emulsion. Pour in the milk and vanilla extract, whisking until smooth and fully incorporated. Using room-temperature eggs and cooled melted butter ensures they combine smoothly without creating lumps or separating.

Pour the wet mixture from Step 4 into the dry mixture from Step 3 and stir gently with a spatula until just combined—do not overmix, as this will develop gluten and make the muffins tough. Fold in the walnuts, strawberries, and rhubarb using as few strokes as possible, being careful not to crush the fruit. The batter should look slightly lumpy rather than smooth.

Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full. Generously top each muffin with the streusel topping from Step 1, pressing it down very gently so it adheres but doesn't compact. The generous topping creates that signature crumbly texture that makes these muffins special.

Bake at 425°F for 15–18 minutes, until the muffin tops are golden and a toothpick inserted into the center of a muffin (avoiding the fruit) comes out clean. Allow the muffins to cool in the pan for 10 minutes to set slightly, then transfer to a wire cooling rack. This cooling-in-pan step allows the structure to stabilize so the muffins hold their shape when moved.