Preheat the oven to 150C/300F/Gas 2.
Place the giblets and stock into a saucepan and simmer for 30 minutes.
Meanwhile, heat a large ovenproof casserole or deep roasting tray on the hob. Add 25g/1oz of the butter and the lardons and fry for 1-2 minutes, then add the button onions and fry for a further 2-3 minutes, or until golden-brown. Remove the bacon and onions and set aside.
Prick the ducks all over with a sharp knife and rub all over with a little salt. Fry the ducks in the casserole until browned all over. Drain off any fat.
Strain the stock into the casserole, bring to the boil, cover with a lid or foil and braise in the oven for one hour.
Return the button onions, bacon to the casserole with the carrots and leeks and braise for a further 15 minutes.
Remove the ducks from the casserole and set aside to rest in a warm place. Spoon off any excess fat.
Return the casserole to the hob and bring to the boil. Add the herbs, peas and lettuce to the sauce and continue to cook until the volume of the liquid had reduced by one-third.
Whisk in the remaining butter and season, to taste, with salt and freshly ground black pepper.
To serve, carve the ducks and arrange on a serving plate, then spoon over the sauce.