Heat oven to 190C/170C fan/gas 5.
Cut small pockets into the skin and flesh
of the lamb by piercing it with the point of
a sharp knife. Stuff each pocket with a
slice of garlic and a few leaves of rosemary,
or scatter with thyme. Put in a roasting
tin, season well with salt and freshly
ground black pepper, then cover with foil.
Peel and thinly slice the potatoes, rinse
under the cold tap and pile into a large
ovenproof dish or roasting tin. Toss with
the onions, remaining slices of garlic and
a good scattering of herbs.
Heat the stock and butter together, then
pour over the potatoes. Cover the dish
with foil and bake in the oven with the
lamb for 1 hr. Uncover, put the lamb on
top of the potatoes and roast uncovered
for 45 mins more. Allow the lamb to rest
before carving, for about 15 mins. Leave
the potatoes in the oven (covered, if
starting to brown too much) until ready