Preheat your oven to 400°F and grease a standard 12-cup muffin tin. While the oven heats, prepare the crumb topping by combining the flour, brown sugar, cinnamon, and chopped walnuts in a small bowl. Cut the cold butter into small cubes and add to the mixture, then use your fingertips to work it together until it resembles coarse breadcrumbs with pea-sized pieces of butter throughout. Set this topping aside—I like to keep it cool, so I sometimes refrigerate it while preparing the batter.

In a large mixing bowl, whisk together the flour, brown sugar, baking soda, baking powder, and salt until evenly combined. In a separate medium bowl, whisk together the oil, egg, room-temperature buttermilk, and vanilla until fully incorporated. This parallel prep ensures both mixtures are ready to combine without overmixing the batter.

Pour the wet ingredient mixture from Step 2 into the dry ingredients and stir until just combined—the batter should be lumpy and slightly undermixed, which prevents tough muffins. Gently fold in the rhubarb pieces, whole walnuts, and lemon zest using a spatula with just a few strokes until evenly distributed. I find that keeping the rhubarb pieces to a consistent 1/2-inch size ensures they distribute evenly and bake through without any tough or raw pieces.

Divide the batter evenly among the 12 prepared muffin cups, filling each about 2/3 full. Sprinkle the crumb topping from Step 1 generously over each muffin, pressing it gently so it adheres to the batter.

Bake at 400°F for 5 minutes to set the outer structure and prevent overmixing issues. Then lower the oven temperature to 350°F and continue baking for 20 minutes until a toothpick inserted into the center comes out with just a few moist crumbs (the rhubarb releases moisture, so don't expect it to come out completely clean). The two-temperature approach helps the muffins bake evenly without over-browning the tops.

Remove the muffin tin from the oven and let the muffins cool in the pan for 10 minutes. This resting time allows them to set and firm up, making them easier to remove without falling apart. Run a thin knife around each muffin to loosen it, then transfer to a wire rack to cool completely.