Grab a cast iron pot and heat over medium heat. Add 1 tablespoon of oil.
Pat the lamb dry and season with a little bit of salt and pepper.
Sear the lamb in the oil until nicely golden on all sides. Should take 2-3 minutes in total.
Take the lamb out and if needed add another tablespoon of oil. Add in the celery and carrots, fry for about 5 minutes until slightly softened and the celery started going translucent.
At this point you add in the tomato puree and let it fry with the vegetables for 1 minute to take the raw taste out of it.
Add in the red wine and deglaze the bottom of the pan. Make sure to scrape off any bits that may have stuck to the bottom. Leave the wine to reduce for 3 minutes.
Add in the stock, water and the herbs. Return the lamb legs to the pot, they should be covered about 2/3 of the way up. If not then add a little bit more water and stock until the correct level.
Turn the heat down to medium low and cover with a lid. Let simmer gently for about 2 hours flipping the meat over after about 1 hour has passed for even cooking.
The lamb is ready when the meat is coming off the bone. Be careful when you take it out of the pot.
At this point you should check if the sauce has the desired thickness. If it is a little bit too thin you can turn the heat up and cook it without a lid for a few minutes to boil off some water. If it is too thick you can add a splash more water to thin it down, adjust seasoning accordingly.