In a large oven and stovetop safe pot, heat the olive oil over medium-high heat. Add the chicken and sear until browned, being careful not to overcrowd the bottom of the pot or the chicken will not brown. Remove the chicken from the pot.
Lower the heat and add the onion, garlic, celery, carrot and herbs, and saute until they begin to soften, about 5-10 minutes. Deglaze the pot by adding the white wine, and using a wooden spoon, scrape any brown bits off the bottom of the pot so they are incorporated into the stew.
Sprinkle the flour evenly over top. Saute for several minutes, stirring well to prevent clumps.
Add the broth, squash, apple and chicken back into the pot and bring to a simmer. Let simmer 15-20 minutes or until the squash and carrot has softened.
While the stew is simmering, make the biscuits. Preheat the oven to 425F. In a large bowl, sift together the flours, baking powder, baking soda, and salt. Using a pastry cutter or 2 knives, cut in the butter until the mixture resembles coarse crumbs. Make a well in the centre and add the butter milk, stirring gently to combine. Gently fold in the sage and cheddar cheese at this point until just combined.
Tip the dough out onto a well-floured surface. Pat the dough into a flat disc, then fold the dough in half and repeat. Roll the dough to 1/2 inch thick and cut out biscuits using a 2 inch biscuit cutter. Brush the top of the biscuits with egg white if desired for extra shine.
Once the vegetables are cooked through, remove from the heat. Stir in the cream and season to taste with salt and pepper. Working quickly, delicately place the biscuits on top of the stew until the surface of the pot is covered, with some room for expansion in the oven.
Place the pot in the oven and bake until the tops of biscuits are browned, about 18 minutes. Remove from the oven and let cool slightly before serving.