Preheat oven to 400 degrees F (200 degrees C).

In a large bowl, combine flour, sugar, baking powder, lemon zest, and salt. Cut in cold butter. Quickly, rub the butter into the dry mixture until the butter is pea-sized. (I do this in a food processor. Just add all the dry ingredients first, cut in the butter, then pulse until the butter is pea-sized).

Mix the egg and buttermilk in a small bowl, and stir into the flour mixture until just moistened. Do not over-mix.

Add frozen blueberries, and stir slightly to incorporate them into the dough.

Turn the dough out onto parchment paper, and use your hands to incorporate any remaining dry ingredients.

Roll or pat dough out into a 1 1/2 inch (4 cm) thick round. Brush with egg yolk or buttermilk then scatter turbinado sugar over the top (if using).

Cut into 8 wedges and place, spaced apart, onto a parchment-lined baking sheet. Feel free to cut into shapes of equal size instead of wedges. 

Bake 15 minutes in the preheated oven, or until golden brown.