• In a large bowl, combine the flour, sugar, salt, and baking powder and mix them well together. Cut the cold butter into cubes and add it to the flour. Use a pastry cutter to cut the butter into small pea-sized pieces. Alternatively, use a food processor to cut the butter into the flour.
• Add in the blueberries and lemon zest and toss to combine.
• Whisk together the sourdough starter, egg, and two tablespoons of heavy cream in a separate bowl.
• Make a well in the center of the flour mixture, then pour in the sourdough mixture. Use a fork to combine it into a shaggy dough that sticks together easily when pressed. If the mixture isn't coming together well, add a tablespoon of milk or cream if necessary, but the dough shouldn't be wet.
• Tip the shaggy dough onto a lightly floured surface and gently push it with your fingers until it is combined into a dough mound. Don’t over-knead the dough, or it will become tough. Press this into an 8-inch circle.
• Cut the circle into eight wedge pieces using a knife or dough scraper. Place the pieces on baking paper and pop them into the fridge while the oven preheats. 
• Preheat the oven to 200°C/400°F.
• Brush the tops of the scones with a bit of heavy cream or milk.
• Bake the scones in the oven for around 20-25 minutes until golden brown around the edges. This timing depends on your oven. Some need longer; some need less. Once baked, leave the scones to cool before glazing.

Lemon glaze

• Add the lemon juice and enough powdered sugar until it makes a thick but pourable glaze. Drizzle it over the cooled scones.