Prepare a sheet tray with parchment paper. Preheat oven to 425 degrees.
Put 2 cups of flour, baking powder and salt in a food processor. Pulse a few times to incorporate.
Add the butter, pulsing until the butter is the size of peas.
Combine the buttermilk and egg in a small measuring cup and beat lightly. Add the scallions.
Slowly add the buttermilk mixture and pulse just until combined. You do not want to over mix!
Now add the cheese, pulsing once or twice, again being careful not to over mix. Finally add the apples, and pulse once or twice, just to combine.
Dump the dough onto a floured surface and knead lightly about 2 times. Roll the dough out into a square until it is about 2 inches thick. With a sharp, floured knife, cut the dough lengthwise in thirds and then across in thirds, making 9 squares. Transfer to a sheet pan lined with parchment paper.
Brush the tops and sides with the beaten egg and water mixture, then sprinkle lightly with some sea salt.
Bake for about 20 minutes, until the tops are golden and the biscuits are cooked through.