Preheat the oven to 210°C. Stuff the cavity of the chicken with the rosemary, reserving 2 teaspoons of the leaves. Place the sage and garlic in the cavity.
Place the chicken, breast-side-down, on a rack in a baking dish and roast for 30 minutes. Turn breast-side-up and roast for a further 30 minutes or until cooked through. Transfer to a platter.
Add the stock, lemon juice, reserved rosemary leaves and 1/4 cup water to the baking dish and stir, scraping up the browned bits. Pour the drippings into a gravy separator, then pour the defatted juices into a small saucepan. Blend the flour with 1/4 cup water, add to the saucepan, and cook over medium heat, stirring, until the gravy is thickened. Serve the gravy alongside the chicken.