Photograph: Jonathan Gregson
Preheat the oven to 190°C, fan 170°C, gas 5 and line the holes of a 12-cup muffin tin with squares of nonstick baking paper or muffin cases. Mix the flour, baking powder and caster sugar in a large bowl. Lightly whisk the milk, olive oil and eggs together in a jug. Pour the egg mixture into the bowl and gently mix with a balloon whisk until half combined.
Add the lemon zest and blueberries and fold in carefully and quickly, just a few times, to combine, using a large spoon – don't overmix. Spoon the mixture into the muffin tins. Sprinkle the muffins with the demerara sugar, if using, and bake on the middle shelf for 25-30 minutes.
Get ahead: best eaten on the day, but they freeze well, too.