Take the thyme leaves and rosemary fronds from the stems and chop finely, along with the parsley.
Zest and juice the lemon.
Put the stuffing ingredients into a mixing bowl and mix.
Remove the lamb from the fridge 30 minutes before you are ready to cook.
Open the saddle of lamb and place it skin down on a board.
Cut 3 or 4 pieces of string about 10 centimetres longer than the meat.
Scoop the stuffing into the middle of the saddle in an even sausage shape.
Roll up the lamb and tie the strings. Trim off any excess.
Peel the onions and cut them into thick slices.
Place the slices in the bottom of the roasting tin.
Put the lamb on top of the onions and rub the remaining oil onto the outside. Season with the salt and pepper.
Preheat the oven to 200 C / 400 F / 180 FAN / Gas 6.
Roast for 20 minutes and then turn the heat down to 190 C / 375 F / 170 FAN / Gas 5 and roast for 30 minutes per 500g or per pound.
Put the lamb on a carving tray and cover loosely with foil to rest for 10 to 20 minutes.
Carve into thick slices and serve with the onions on the side.