Spread saddle of lamb with softened butter and sprinkle with salt, pepper and crushed rosemary, to taste.
Place saddle in a moderately hot
Remove roast from oven; discard fat from pan; add well-flavoured beef stock and a beurre manié (
Slice each baby marrow lengthwise into 4 or 5 slices, without cutting all the way through, to make a fan shape. Place a thin slice of tomato in each opening. Place partially roasted saddle of lamb in an oiled baking tin on which you have scattered finely-chopped onion, sprigs of thyme, garlic and salt and freshly-ground black pepper, to taste. Surround with stuffed marrows; sprinkle with a little olive oil and continue to roast for about 45 minutes, or until meat is tender (allow about 15 minutes per pound in all).
Place lamb on a large heated serving dish; place stuffed baby marrows at one end of the dish and sautéed potatoes at the other. Sprinkle vegetables with finely-chopped mushrooms, parsley and truffles, which you have sautéed in butter. Garnish with watercress. Serve gravy separately.