Using a mixer fitted with a paddle attachment, combine flour, baking powder, salt and black pepper in a large bowl on low speed. With mixer running, gradually add cubes of butter until the mixture is crumbly and studded with flour - butter bits about the size of small peas. Add grated cheese and mix just until blended. (This can also be done by hand: In a large bowl, stir together flour, baking powder, salt and black pepper. Gradually cut in butter with a pastry blender or two knives until mixture resembles small peas. Stir in cheese.)
Add green onions, bacon and 3/4 cup of the buttermilk to flour and cheese mixture. Mix by hand just until all ingredients are incorporated. If dough is too dry to hold together, use remaining buttermilk, adding 1 Tbsp at a time, until dough is pliable and can be formed into a ball. Stir as lightly and as little as possible to ensure a lighter-textured scone. Remove dough from bowl and place it on a lightly flours flat surface. Pat dough into a ball. Using a well-floured rolling pin, flatten dough into a circle about 8 inches wide and 1/2 inch thick. Cut dough into 8 to 10 equal wedges, depending on the size scone you prefer.
Whisk egg and water in a small mixing bowl to combine. Brush each wedge with egg wash. Place scones on a Silpat or parchment paper-lined baking sheet and bake for 18 to 20 minutes, or until golden brown and no longer sticky in the middle. Serve warm.