Season saddle of lamb with salt and freshly ground black pepper. Tie 12 sprigs of fresh rosemary to it and sprinkle lightly with olive oil. Roast lamb in usual way and serve with sauce.
Sauce
Bring 4 sprigs fresh rosemary to a boil in ½ pint water. Reduce water to about 4 tablespoons over a high heat.
Heat gravy in the top of a double saucepan; add 2 tablespoons reduced rosemary water to this and then, over hot but not boiling water, beat in diced softened butter until sauce is thick and smooth. Add cognac, and season to taste with salt and freshly ground black pepper.