Wipe lamb with a cloth soaked in vinegar. Pad dry with absorbent kitchen paper. Rub the lamb thoroughly with salt and pepper, grated lemon peel, garlic and marjoram.
STUFFING: Heat olive oil in a frying pan. Sauté onions until golden, about 5 minutes. Add breadcrumbs, garlic, feta cheese, parsley, grated lemon peel, salt and beaten egg. Blend thoroughly.
Stuff pocket of lamb. Fasten opening securely with skewers. Brush lamb with olive oil and place on grid of roasting pan.
ROAST TOMATOES: Sprinkle tomatoes with mustard powder, garlic, sugar, seasoning and olive oil. Arrange tomatoes around lamb.
Roast lamb at 180 ºC, calculating 15 to 20 minutes for each 500 g lamb (rare to medium-rare), plus 20 minutes at end.
Cover with foil for first 30 minutes.
GRAVY: Wash, pat dry and lightly flour lamb bones. Place in a roasting pan. Roast at 200 ºC for 20 to 25 minutes to brown thoroughly. Add carrots, tomato, onion and celery. Braise for 5 to 10 minutes.
Add water and wine. Stir briskly and continue cooking, uncovered, at 170 ºC for another 45 to 60 minutes. Remove from stove, strain stock and boil, uncovered, over high heat in a saucepan to thicken further, if necessary. Season.
LENTILS: Toss in olive oil, garlic, bay leaves, lemon juice and seasoning. Serve lamb and gravy with steamed baby marrows, tomatoes and lentils.