In a large bowl, whisk the flour, sugar, baking powder, and salt together.
With a pastry blender or two forks, cut in the cold butter cubes until pea-sized pieces remain.
Using a spatula, fold in the diced strawberries.
Create a well in the middle of the mixture and pour in half of the buttermilk. Stir to incorporate. The dough will be dry but subtly moistened.
Add the remaining buttermilk. Gently mix it in just until moistened and incorporated. Remove the dough from the bowl and place it onto a lightly floured surface.
Gently knead the dough until it comes together. (Do not over-mix!) Form the dough into a flattened disk.
With a bench scraper portion the dough into 8 triangles. Place the triangles onto a parchment-lined baking sheet.
Refrigerate the scones for at least 30 minutes.
While the scones are chilling, preheat the oven to 400° F.
In a small bowl, whisk together the egg and a tablespoon of either water or cream. Brush the egg wash onto each scone. (You will not use all the egg wash.) Sprinkle each scone with turbinado sugar.
Bake for 20-25 minutes, or until golden brown.