Preheat your oven to 350°F and line a 12-cup muffin pan with paper liners. While the oven heats, prepare the crisp topping by combining the brown sugar, oats, cinnamon, and cold butter cubes in a small bowl. Use your fingertips to rub the butter into the dry ingredients until the mixture resembles coarse breadcrumbs with some pea-sized pieces of butter remaining—this creates the texture that makes the crisp topping crispy and buttery. Set aside until ready to use.

In a large mixing bowl, whisk together both flours, 3/4 teaspoon cinnamon, baking soda, and salt. Add the sliced rhubarb and 3/4 cup brown sugar to the dry mixture, gently folding everything together with a spatula. The rhubarb should be evenly coated with the dry ingredients—this helps distribute it throughout the muffins and prevents it from sinking to the bottom.

In a separate bowl, whisk together the room-temperature egg, yogurt, and oil until well combined and the egg is fully incorporated. I find that using room-temperature eggs helps them blend more smoothly with the other wet ingredients. Pour the wet mixture into the dry rhubarb mixture and gently stir with a spatula until just combined—stop as soon as you don't see streaks of flour. Overmixing will develop gluten and make the muffins tough, so restraint is key here.

Divide the batter evenly among the 12 lined muffin cups, filling each about two-thirds full. Top each muffin with approximately 1 tablespoon of the crisp topping from Step 1, distributing it evenly across the surface. Don't press it down—let it sit loosely on top so it stays crispy and doesn't sink into the batter during baking.

Bake in the preheated 350°F oven for 20 minutes, until the tops are golden brown and a toothpick inserted into the center of a muffin comes out with just a few moist crumbs. Allow the muffins to cool in the pan for 15 minutes before removing them from the liners—this resting time lets them set and makes them easier to handle without falling apart.