Preheat oven to 400 degrees and line a baking sheet with parchment paper.
Using a stand mixer with a paddle attachment, combine flour, baking powder, pepper and salt.
With the mixer running, gradually add the cubes of butter, mixing until mixture is crumbly and there are no pieces of butter larger than a small pea.
Add cheddar cheese, mixing just until blended.
Add green onions, bacon, diced jalapeno, and 3/4 cup buttermilk. Mix by hand just until ingredients are incorporated, adding extra buttermilk a little at a time until dough comes together into a ball. Don't work the dough any more than necessary. The less you handle it, the more tender and light the scones will be.
Place dough on a lightly floured surface and flatten into a circle that is about 1/2-inch thick and 8 inches wide.
Cut dough into 8 wedges and place on prepared baking sheet.
Whisk the egg and water together in a small bowl and brush tops of scones with egg mixture.
Arrange 2 jalapeno slices on top of each scone.
Bake for 20-24 minutes, or until golden brown. Serve warm.