Preheat oven to 350 degrees F.

Lay the meat, skin-side-down, on a board and cut out any lumps of fat. Cover with a piece of wax paper and pound to a fairly even thickness with a meat mallet or the end of a rolling pin. Remove the paper. Lightly sprinkle the meat with salt and pepper, then spread the herb mixture over the surface. Roll the meat up tightly and secure it in 2 or 3 places with kitchen twine.

Place meat on a rack in a roasting pan. Brush all over with the olive oil, then coat with mustard.

Roast the meat for 1/2 hour, then baste it well, sprinkle with the brown sugar, and slip the onion slices under it to brown and flavor the juices. Roast for a further 1-1/2 hours, basting once, until the meat is a rich brown.

Lift the meat onto a dish, cover loosely with aluminum foil, and keep warm in a warming oven or in the regular oven turned down to its lowest setting, while you prepare gravy.

To make the gravy, skim off all but 2 teaspoons of fat from the pan and remove the onion. Add the cornstarch mixed to a smooth consistency with the water, the crumbled stock cube, the sugar, and a pinch of sea salt and black pepper. Simmer on top of the stove until thick and glossy, about 3 minutes.