Preheat oven to 375°.
In a large bowl, combine the flour, baking powder, sugar, and salt. Cut or grate the butter into the flour mixture. Then, stir in the sage, apple and gruyere until well distributed. Set aside.
In a smaller bowl, combine the 2 eggs and buttermilk. Whisk until well combined. Make a well in the center of the flour mixture, pour in the egg mixture and stir until it comes together. Whisk the additional egg in a small bowl combined with 1 tablespoon buttermilk. Set aside.
Turn out on a lightly floured counter and knead a few times until it holds it's shape, adding additional flour if needed. Divide into two equal portions and roll into an 8 or 9 inch circle. Dust the bottom of the circles with flour.
Cut each circle into 6 to 8 triangles. Place the triangles on a parchment lined baking sheet. Lightly brush with the egg wash, then bake for 15-25 minutes at 375°, or until golden and cooked through.
Cool on a wire rack until cool enough to handle. Serve.