1) Heat a large Dutch oven with a tight-fitting lid over medium-high heat. Pour in enough oil to fill the pan about 1/2cm deep. Spread the flour out on a plate or piece of greaseproof paper. Season half of the lamb with one teaspoon of the salt and some pepper. Dredge the lamb in the flour and shake off the excess. Cook, uncovered, stirring only occasionally, until well browned, about eight minutes. Using a slotted spoon, transfer the lamb to a plate. Repeat with the remaining lamb. Discard the oil and wipe out the pan.
2) Preheat the oven to 140°C / gas mark 1. Return the pot to the stove and melt the butter over medium-high heat. Add the onion and cook, stirring, until lightly browned, about five minutes. Stir in the garlic and tomato puree and cook until fragrant, about two minutes.
3) Return the lamb to the pot, add the water or stock, and bring to a simmer. Using a piece of kitchen string, tie together the parsley, thyme and bay leaves and add the bundle to the pot. Stir in the remaining two teaspoons of salt and pepper. Cover and transfer to the oven. Stew the meat until tender, about 1 1/2 hours.
4) Remove the Dutch oven from the oven. Skim the fat from the cooking liquid with a ladle. Add the potatoes, carrots, celery and tomatoes, and bring to a simmer. Cook the stew, uncovered, on top of the stove until the liquid has thickened and the vegetables are tender, about one hour. Remove and discard the herb bundle. Stir in the vinegar.
Divide among bowls and serve. immediately.