Heat the oven to 180C (160C fan)/350F/gas 4. Heat some oil in a large casserole dish over a medium heath and cook the onions until soft and golden, but not browned
Meanwhile, peel the floury potatoes and one carrot and slice thinly (preferentially with a mandolin)
Tip the onions into a bowl, add a bit of oil to the pan and brown the lamb
Now line the base of the pan with sliced potato. Tip the onion back into the pot, along with the lamb and thyme, season and pour in enough vegetable stock to barely cover the meat
Bring to a bare simmer, then cover and transfer the casserole to the oven for an hour and a half
Just before the end of this time, peel the carrots and cut into large chunks and cut the salad potatoes in half
Remove the casserole form the oven, remove the meat and with a stick blender liquidize the vegetables. You can skip this step for a more rustic stew
Now put the carrots at the bottom of the pot and put the potatoes and lamb on top. Ad some boiling water if necessary to ensue that everything is barely covered
Leaving the lid slightly ajar, cook for another 30 minutes, by which time the meat should be very tender.