For the ossobuco, peel and chop the onions and garlic. Clean or peel, wash and chop the carrots and celery. Heat 4 tablespoons of oil in a large frying pan. Fry the carrots, celery, onions and garlic until slightly browned. Stir in tomato paste and sauté briefly. Remove the roaster from the heat and set aside
Wash the meat, dab dry and cut the almost transparent silver skin more often, but do not cut into the meat. Wrap the leg slices tightly with kitchen twine. Season with salt and pepper. Turn in flour and tap off the excess flour. Heat 6 tbsp. oil in a large frying pan in portions. Fry the meat on both sides for about 3 minutes at medium heat until golden brown. Remove the meat and place it on the roasted vegetables in the roasting pan.
Carefully drain the oil from the pan and deglaze the remaining roast with wine.
Wash and quarter the tomatoes. Wash parsley, shake dry and chop coarsely with stalks. Pour beef stock into the wine stock. Add parsley, thyme and bay leaf, bring to the boil once. Season with salt and pepper
Pour the sauce with the tomatoes over the meat, place the lid on top. Braise in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 2-3 hours until the meat is tender. Meanwhile, pour your own roast stock over the meat about every 30 minutes
For the Gremolata, wash the lemons, dab dry and finely grate the rind. Wash parsley, dab dry, remove leaves and chop finely. Peel garlic and chop very finely. So that the flavours combine well, chop everything once together on a board. Mix Gremolata in a small bowl
For the puree, peel the celery tubers, dice them coarsely and cook them covered with laurel in salted water for about 20 minutes until soft. Wash the lemon, dab dry and finely grate the peel. Squeeze the lemon. Drain the celery and warm the milk. Pour half to the celery and puree briefly with a hand blender. Gradually stir in the rest of the milk until the puree is creamy. Stir in butter and crème fraîche. Season to taste with salt, pepper, nutmeg, lemon peel and lemon juice
Remove the osso buco with the tomatoes. Pour the roast stock through a fine sieve into a pot. Bring the sauce to the boil and reduce for about 5 minutes. Season to taste with salt and pepper. Arrange the ossobuco with the tomatoes and the sauce. Sprinkle Gremolata over it. Add the celery puree