Preheat the oven to 375 degrees F. Place chicken in a shallow baking dish and brush with olive oil. Sprinkle with the chopped fresh sage, salt, and a generous amount of pepper. Bake for 40 to 45 minutes, or until the chicken is cooked through. Set the chicken on a plate and cover with foil. Remove the fat from the pan-drippings and then place the pan on the stove. Add the chicken stock/flour mixture and bring to a boil, scraping the bottom of the pan to dislodge the flavourful brown bits. Simmer until thickened and season to taste. Divide the chicken among plates and serve the gravy in a sauceboat alongside for drizzling on top.