Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large bowl, whisk together the flour, granulated sugar, baking powder, salt, and cinnamon until well combined.
In another bowl, mix the melted butter, egg, and buttermilk until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Be careful not to overmix; a few lumps are okay.
Fold in the chopped rhubarb until evenly distributed throughout the batter.
In a small bowl, combine the brown sugar, rolled oats, and nutmeg to create the topping.
Spoon the muffin batter into the prepared muffin tin, filling each cup about two-thirds full. Sprinkle the topping mixture evenly over the batter in each cup.
Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.