Mix the flour and the salt in a shallow dish. coat the Veal pieces with the flour mixture and shake off any excess flour. In a large frying pan, heat the oil and butter on medium heat until the butter melts. Lightly fry the pieces of veal on both sides until they are golden brown. Place the veal aside in a plate and cover to keep heated.
In a large frying pan, add the onions, carrots and celeri. Cook while stirring (around 5 minutes) until the vegetables begin to sweat. Add the garlic and continue stirring for about 30 seconds. Pour in the wine while scrapping the bottom of the pan to loosen all the particles. Turn up heat, bring to a boil and let boil for 2 -3 minutes.
Add the veal pieces to the sauce. Add the chicken stock, tomatoes (with their juice), thyme and a bay leaf and bring all to a boil. Cover and place in pre-heated oven for 1 ½ hours, (turning the pieces of veal from time to time). Uncover for the final 30 minutes and continue cooking until the sauce has thickened. Adding a bit of water if the sauce thickens too quickly.