Preheat oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease well.
In a small bowl, combine oats and milk. Let sit 10 minutes to soften.
In a large bowl, whisk together flour, brown sugar, baking powder, baking soda, salt and cinnamon (if using).
In a medium bowl, beat the egg lightly, then stir in melted butter (or oil) and vanilla extract.
Add the soaked oats (with any remaining milk) to the wet ingredients and stir until blended.
Pour wet mixture into dry ingredients. Fold gently with a spatula until just combined—do not overmix.
Fold in blueberries and lemon zest, reserving a few berries for topping.
Divide batter among muffin cups, filling each about ¾ full. Top with reserved berries.
Bake 18–22 minutes, until a toothpick inserted in the center comes out clean and tops are golden.
Let muffins cool in pan 5 minutes, then transfer to a wire rack to cool completely or serve warm.

For a streusel topping, mix 2 tbsp oats, 2 tbsp brown sugar and 1 tbsp melted butter; sprinkle on batter before baking.
Swap up to half the flour for whole wheat for a nuttier taste.
Stir in ¼ cup chopped nuts or shredded coconut for extra texture.
Store in an airtight container at room temperature for up to 2 days or refrigerate up to 5 days.
Freeze cooled muffins in a single layer, then transfer to a freezer bag for up to 3 months; thaw or reheat before serving.