Heat oven to 375 degrees F. Prepare a standard muffin pan with paper liners, and set aside.

To make the topping, in a large bowl, mix together the oats, butter, brown sugar, flour, cinnamon, and baking soda until well combined and the butter resembles coarse crumbs, the size of peas. Set aside.

In a medium bowl, whisk or sift together the flour, baking powder, salt, and nutmeg. Set aside.

In the bowl of a stand mixer, cream together the butter and sugar until light and fluffy, about 5 minutes. Add in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract. Add the dry ingredients alternately with the buttermilk, making sure to start and end with the dry ingredients. Mix until just incorporated, being careful not to over-mix. Fold in the blueberries.

Using an ice cream scoop, portion out the batter into the prepared muffin pan, filling them 3/4 of the way full. Sprinkle each with about 1 tablespoon topping and give them a gently tap with your fingers to slightly pack down the crumb topping. Bake until golden brown, puffed up, and a toothpick inserted in the middle, comes out clean, about 15 to 18 minutes. Remove from oven and let cool in the pan for about 10 minutes before transferring to a wire rack to cool completely. Leftovers can be store in an airtight container at room temperature for up to 2 days or in the fridge for up to 3 days. Enjoy!