1Preheat the oven to 160°C. Place the flour into a large bowl and season well with salt and pepper. Dust the lamb shanks in the flour mixture, shaking off any excess.
2Heat 1 tbsp oil in a large, heavy-based, ovenproof saucepan over medium-high heat. Cook the shanks for 5 minutes, turning, until browned. Remove and set aside.
3Pour in the wine and boil for 5 minutes to cook off the alcohol. Return the shanks to the pan with the eschalots and trussed thyme. Pour in enough water to submerge the shanks and bring up to a simmer. Cover the surface with a piece of baking paper, then cover with a lid and place in the oven for 1.5 hours.
4Carefully remove the pan from the oven and turn the shanks over, topping up with water if necessary. Add the mushrooms and cover with the baking paper and lid, then return to the oven for a further 1.5 hours or until the shank meat is falling from the bone.
5Meanwhile, heat the remaining 2 tsp oil in a small frying pan over medium heat. Toast the remaining thyme until bright green, then remove and set aside.
6Remove the shanks from the pan and place in a bowl, loosely covered with foil and set aside to rest.
7Place the pan over high heat. Reduce the sauce for 10-12 minutes or until thickened and glossy.
8Divide polenta among bowls and top with a shank, mushrooms and eschalots. Spoon over the sauce and scatter with toasted thyme to serve. Season to taste with salt and pepper.