Melt the butter in a large saucepan over medium heat. Add the onion and garlic and sauté, 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 1½ 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.