1) Beat the escalopes until thin and cut into short strips. 2) Melt half the butter in a frying pan, add the onion and mushrooms and fry until soft. 3) Stir in the tomato purée and flour. Cook, stirring, over a low heat for 2-3 minutes. 4) Remove from the heat. 5) Melt the remaining butter in a separate pan, add the veal and fry over a high heat, turning until evenly browned. 6) Add the meat to the sauce and stir well. 7) Add the soured cream, salt, pepper and lemon juice to taste and serve immediately.