Preheat the oven to 375 F and line a muffin tin with muffin wrappers.
In a large mixing bowl, combine the oats and the buttermilk. Let them sit while you prep the other ingredients, or up to 30 minutes to soften the oats.
In another bowl, sift in the whole wheat flour, baking powder, baking soda, and salt.
Add the melted butter, brown sugar, egg, lemon juice, and zest, and vanilla extract to the oat mixture, stir until combined.
Add the flour mixture to the oat mixture and stir until just combined. Careful not to overmix them! Then fold in the blueberries.
Evenly divide the batter into a muffin tin. Bake for 15-20 minutes or until a toothpick comes out clean.
Place on a wire rack to cool, then drizzle with Lemon Blueberry Glaze.
Lemon Blueberry Glaze
Add the blueberries, lemon juice, vanilla extract, and buttermilk to a food processor or blender and mix on high until completely combined. Then transfer the mixture to a small mixing bowl using a strainer to catch any blueberry remains from the skin.*
Whisk in the powdered sugar until completely combined. You can add more sugar to make the glaze thicker or more buttermilk to thin it out.
Notes
If you don't have a food processor or blender, the blueberries can be muddled but make sure to mash them very well.
The glaze can be made without a strainer if you don't have one, but i recommended to catch the remains of the blueberry skin.