Preheat the oven to 350°F (170°C or 160°C for fan ovens).
Grease and flour a 12 cup muffin pan or line with paper liners.
Make the streusel:
Cut the cold butter and the flour together using a pastry blender or two dinner knives.
When the mixture resembles coarse crumbs, stir in the Demerera sugar, ginger and oats.
Make the muffin batter:
Cream the melted butter and sugar in a medium bowl.
Beat in the egg.
Add the vanilla and milk and mix thoroughly.
In a separate large bowl, stir together the flour, baking powder, ginger and salt.
Add the chia seeds and the lemon rind and stir together.
Toss in the blueberries and stir gently so that they are lightly covered with the flour mixture.
Add the wet ingredients to the blueberry flour mixture all at once, folding in gently just to mix. The batter will be a bit lumpy but there should be no dry flour still visible. Don't stir too enthusiastically or you will have batter that is purplish blue all the way through. (Not a disaster but it looks better if that doesn't happen!)
Spoon the batter carefully into 11 to 12 large muffin cups. (I like to use an ice cream scoop to do this.)
Sprinkle each muffin with the streusel mixture.
Bake for 20 to 25 minutes or until the muffin tops spring back when touched gently and a skewer inserted into a muffin comes out virtually clean.
When cooked, remove the muffins from the pan and cool on a wire rack.
These are delicious warm from the oven but do allow them to cool slightly first to avoid being burned by the molten hot blueberries fresh from the oven!
The muffins will keep for 2 days at room temperature in a sealed container. If you want them to keep longer - or if it is very hot where you are - store them in the fridge. Just remember to bring them back to room temperature or warm gently before eating.