Melt the butter in a large saucepan over medium heat. Add the onion and garlic and saute, stirring often, for 15 to 20 minutes, until the onion becomes browned and starts to caramelize. Sprinkle on the flour and cook, stirring constantly, for 1 1/2 minutes. Stir in the chicken stock, Cognac, sage leaves, bay leaves, 2 teaspoons salt (depending on the saltiness of the chicken stock), and 1 teaspoon pepper. Bring to a boil, lower the heat, and simmer for 20 minutes, stirring occasionally. Set aside at room temperature for 1 hour and strain, pressing the solids lightly and then discarding them. Refrigerate until ready to use.
After the turkey is cooked, remove it to a carving board to rest while you finish the gravy. Place the roasting pan on the stovetop over medium heat and add the wine. Bring to a boil, lower the heat, and simmer for 2 minutes, stirring and scraping up all the bits clinging to the bottom of the pan. Slowly whisk the gravy base into the pan. Simmer for about 5 minutes, until the gravy is smooth and slightly thickened. Taste for seasonings and serve hot.