Pre-heat the oven to 180 C. Slice the onion, and add to a medium oven-proof pan. Add 1 tbsp tomato puree, 1 tbsp plain flour, 1 garlic clove, chopped, 30 ml balsamic vinegar, 200 ml red wine, 1 tsp dried rosemary, and 300 ml water. Stir to mix thoroughly. Nestle in the lamb shanks, and place on a high heat and bring to the boil. Once boiling, transfer to the oven. Turn the oven down to 150 C and leave for 2 hours.