Preheat the oven to 400?F (204?C) and line muffin cups with paper liners (8 for big muffins, 10 for standard, or 22 for mini).

Lightly grease or spray the tops of the muffin tin with oil to prevent sticking.

Remove the zest from the lemon and add it to a small bowl with the sugar, rubbing until fragrant.

Whisk the flour, lemon sugar, baking powder, and salt in a large bowl.

In a measuring jug, combine the oil, egg, and enough milk to reach the 1-cup line, then whisk in the vanilla extract.

Add the wet ingredients to the dry ingredients and stir gently until combined, being careful not to overmix.

Fold in the blueberries and about 2 tablespoons of fresh lemon juice gently.

Divide the batter between muffin cups, sprinkle sugar on top, and optionally add a thin slice of lemon.

Bake for 15 to 20 minutes until the tops are no longer wet and a toothpick comes out with crumbs, not wet batter.

Transfer to a cooling rack.