Clean, wash and chop the carrots and celery. Drain the tomatoes, collect the juice. Chop the tomatoes. Peel and chop onion. Wash leg slices, dab dry and cut the skin 2-3 times around the leg.
Season leg slices with salt and pepper and tie them into shape with kitchen string. Heat the oil in a frying pan, fry the leg slices in it vigorously and take them out. Sauté the diced onions in the frying fat. Add vegetables, tomato juice and paste and flour and fry briefly.
Stir in broth and wine. Place meat on top. Cover and braise in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 1 1/2 hours. Peel garlic for the Gremolata. Wash parsley, shake dry and pluck off the leaves.
Finely chop the parsley and garlic. Wash the lemon, grate dry and thinly grate the peel. Mix garlic, parsley and lemon peel. Sprinkle Gremolata over the meat and serve Osso Buco. Ciabatta is delicious with it.