Prep the butter. The trick here is to dice the butter small, into 1/4" cubes. Place the diced butter in a bowl and transfer to the freezer while you're getting the rest of the ingredients together.
Mix the dry ingredients. In the bowl of a stand mixer, whisk together the flour, sugar, salt, pepper, baking powder, and baking soda. Mix in the minced sage and grated gruyere cheese.
Mix in the butter. Remove the butter from the freezer and add it to the bowl of dry ingredients and toss well. Mix with the paddle attachment on medium-low until the mixture resembles clumpy sand with pea-sized pieces of butter.
Mix in the wet ingredients. With the mixer running, stream in the buttermilk and heavy cream and mix just until the dough clumps together. Transfer to a lightly floured work surface and press the dough into a 8×11 rectangle.
Laminate the dough. Fold the rectangle into thirds, like a letter, and press/roll the dough into a 1-inch thick rectangle. Repeat folding and rolling 3 more times. Press or roll the dough out into a 9" square that's about 1" thick.
Cut and chill. Using a sharp knife, cut the dough into 9 equal squares or use a round biscuit cutter dusted with flour to cut out circles. If you're using a circular cutter, gather the dough back up and continue to cut circles until no dough remains. Transfer the biscuits to a parchment-lined baking sheet and refrigerate for 45 minutes.
Preheat an oven to 450 F.
Bake. Brush the tops with heavy cream and sprinkle with flaky salt. Bake for 18-20 minutes, until puffy and golden brown.