Put the veal or pork, the spring onions, parsley, thyme and garlic into a bowl, season well and mix. Fill the crown with the stuffing, then weigh it and calculate the cooking time, allowing 25 minutes per 500 g, plus 30 minutes.
Put the lamb into a roasting pan, rub all over with the oil, sprinkle with rosemary and season well. Cover the top half with foil and roast in the centre of the oven for the time calculated, basting occasionally. Remove the foil for the last 20 minutes to brown the top.
When the crown is uncovered, put all the vegetables into a wide frying pan with the butter, brown sugar and chicken stock or water. Season with salt and bring to the boil, then reduce the heat and cook gently, partly covered, until the vegetables are tender and the liquid has almost evaporated – shake the pan often towards the end of the cooking time to prevent the vegetables from sticking to it. Remove from the heat, cover and keep hot.
Carefully transfer the crown from the roasting pan to a heated serving plate, cover and keep warm. To make the gravy, skim all but 2 tablespoons of fat from the roasting pan and stir in the flour and veal or chicken stock. Bring to the boil, stirring and scraping any browned residue off the bottom of the pan. Strain the gravy through a sieve into a saucepan and simmer for 5 minutes. Season to taste.
Trim the crown roast with cutlet frills and surround with the glazed vegetables. Serve with the hot gravy and buttered new potatoes. To carve, cut downwards between the bones and serve with a spoonful of the stuffing.