Into a large pot, put chicken, onions, carrots, celery, chopped celery leaves, parsley, sage, pepper and 14 cups cold water. Bring to a boil, skimming off any foam/impurities that surface. Reduce heat and simmer until chicken is tender, about 1 to 1½ hours, depending on the size of your chicken. Transfer chicken to a bowl or plate and let cool briefly. Remove the meat from the bones and set aside. Discard the skin and return all the bones back to the pot. Cover meat and refrigerate. Increase heat and continue simmering stock until rich and golden, about 2½ more hours. Strain stock, discard solids and let cool. Refrigerate overnight.

The next day, remove solid layer of fat from stock. Transfer to a medium pot and bring to a boil, then reduce to a lively simmer and reduce to about 5 or 6 cups. (My stock was reduced to almost that amount already, so use your judgement.)

Preheat oven to 475º. Cut/shred meat into large pieces and place in a large baking dish. Whisk flour and 1 cup water until smooth, then whisk into simmering stock, whisking constantly until thickened, about 5 minutes. Season to taste with salt (don't be shy!). Ladle gravy over chicken and bake until hot and bubbling, about 10 minutes.

Make the biscuits: Whisk flour, sugar, baking powder, salt and baking soda in a large bowl to blend. Using fingertips, rub chilled butter into dry ingredients until mixture resembles coarse crumbs (or, if you prefer, use a pastry cutter). Add buttermilk and stir to combine and evenly moistened. On a floured surface, turn out dough and press by hand to about ½-inch thickness. Use a 3-inch round cookie cutter to cut 12 rounds.

Remove chicken/gravy from oven and arrange biscuits on top in a single layer. Return to oven and bake until biscuits are golden brown, about 15 to 20 minutes.

Set chicken pie on a wire rack to cool at least 10 minutes before serving.