Preheat oven to 360° F (180° C)
Heat the olive oil in a large Dutch oven over medium-high heat. In the meantime, trim any excess fat from the lamb and cut it into chunks if this hasn't already been done. Once the olive oil is hot, add the lamb pieces and brown them on all sides.
While the lamb is browning, peel and cut the carrots and onion into large dice. Then peel the garlic cloves, slice them lengthwise, and remove the germ from each clove.
Add the carrots, onion and garlic to the Dutch oven and cook for 5 minutes.
Sprinkle the arrowroot over the mixture. Stir well.
Add the tomato paste and stir again.
Pour in the water (add water to cover) and stir again. Then season with salt and pepper and add the rosemary sprig.
Cover the pot and put it in the oven for 25 minutes. In the meantime, peel and quarter the potatoes and turnips.
Remove the Dutch oven from the oven and add the potatoes and turnips. Cover and return to the oven for another 35 minutes.
In the meantime, bring a saucepan of salted water to a boil. Once the water is boiling, add the green beans and cook for about 6 minutes if fresh, 10 minutes if frozen.
Once the green beans are cooked, drain them, add them to your Lamb Navarin, add some fresh parsley leaves and serve. Voilà!