Preheat your oven to 350°F and grease two loaf pans (or one 9x13 inch baking dish) with butter or cooking spray. While the oven preheats, prepare the streusel topping by combining the flour, brown sugar, cinnamon, and salt in a small bowl. Cut the cold butter into small cubes and work it into the dry mixture using your fingertips until the texture resembles coarse breadcrumbs. Set this topping aside—having it ready means you can assemble the bread quickly once the batter is mixed.

In a large bowl, whisk together the egg, canola oil, buttermilk, and vanilla extract until well combined and smooth. This creates an emulsion that will help distribute moisture evenly throughout the bread. The whisking also incorporates a bit of air, which contributes to a tender crumb.

In another bowl, whisk together the brown sugar, flour, salt, and baking soda until evenly distributed. Pour the wet ingredient mixture from Step 2 into the dry ingredients and stir gently until just combined—don't overmix, as this keeps the bread tender. I like to use a rubber spatula and fold the mixture rather than stir vigorously, which helps maintain a delicate crumb structure.

Gently fold the diced rhubarb and coarsely chopped pecans into the batter from Step 3 using a rubber spatula, mixing just until evenly distributed. The rhubarb's tartness pairs beautifully with the brown sugar's depth, and the pecans add a satisfying crunch. Be gentle to avoid crushing the rhubarb pieces, which will help them stay distinct in the finished bread.

Divide the batter evenly between the prepared pans (or pour into the single baking dish), smoothing the tops gently. Sprinkle the streusel topping from Step 1 evenly over the batter, pressing it down very slightly so it adheres. Bake at 350°F for 40–45 minutes, until a toothpick inserted in the center comes out clean or with just a few moist crumbs. The top should be golden brown.

Let the bread cool in the pans for 10 minutes—this resting time allows the structure to set so it won't fall apart when you remove it. Turn the loaves out onto wire racks to cool completely. The rhubarb bread is wonderful served slightly warm with a cup of coffee or tea, and it keeps well wrapped at room temperature for several days.