Melt the butter on the stovetop in a Dutch oven or other oven-safe sauté pan with high sides. Stir in the onion and celery, then cover the pan and cook them for 7 to 8 minutes over medium heat, stirring occasionally. Add the flour, stirring for 1 to 2 minutes to lightly brown it.
Whisk the chicken stock into the pan. When it starts to thicken, whisk in the milk. Add the sage, thyme, chicken, and vegetables, continuing to stir until the mixture is heated through, about 5 to 7 minutes. Add salt and pepper to taste.
Remove the pan from the stove top and heat the oven to 375°F Meanwhile, make the biscuit topping by combining the flour, baking powder, sugar, and salt in a mixing bowl. Add the butter and use your fingertips to rub it into the dry ingredients. Add the milk and stir briskly, just until the dough pulls together.
Flour your work surface and turn the dough onto it. Using floured hands, knead the dough two or three times, then flatten it to about 1/2 inch thick. Using a small round cutter, cut the dough into biscuits and place as many as will fit, barely touching, on top of the filling. (You can bake any extras separately, on a lightly greased pie plate, for about 15 minutes.).
Bake the potpie until the biscuits are golden brown and the filling is bubbly, about 20 to 30 minutes. Then let it cool for 5 to 10 minutes before serving it.