Heat your oven to 200°C (400°F) and prepare a 12-cup muffin tin by greasing it or inserting muffin liners. Slice the rhubarb into thin 1/4-inch pieces and chop the 2 oz of pecans into small pieces; set both aside. Halve the 18 pecans for garnishing and set aside separately. Having everything prepped and your oven ready ensures smooth assembly and prevents the batter from sitting idle.

In a large bowl, whisk together the brown sugar, vegetable oil, eggs, and vanilla until well combined. The sugar should dissolve slightly and the mixture should be smooth and emulsified. Gently stir in the buttermilk until fully incorporated—avoid overmixing at this stage. I like to use a gentle hand here because we want to keep things light; aggressive stirring can develop gluten too early.

In a second bowl, whisk together the flour, baking powder, baking soda, salt, and nutmeg until evenly distributed—this ensures the leavening agents are spread throughout and won't create odd pockets of dense muffin. In a third small bowl, combine the 1/3 cup sugar, cinnamon, and butter, stirring until the mixture resembles wet sand with small clumps. This crumbly topping will add texture and sweetness to the finished muffins.

Fold the rhubarb and chopped pecans into the wet mixture from Step 2, then gently fold in the dry ingredient mixture from Step 3 until just combined—overmixing will make tough muffins, so stop as soon as you don't see streaks of flour. Divide the batter evenly among the muffin cups, filling each about 3/4 full. I always use a spring-loaded ice cream scoop for this because it ensures consistent-sized muffins and even baking.

Sprinkle about 1 teaspoon of the cinnamon-sugar topping from Step 3 over each muffin, then crown each with a halved pecan. Bake for 20 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs—the rhubarb will release moisture, so don't overbake or the muffins will dry out.

Allow the muffins to cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely. This initial rest in the pan lets them set slightly while still warm, making them easier to remove without crumbling.