Put the onion, carrot, garlic, stock or water into a saucepan with the chicken breasts and bouquet garni. Bring to a boil, cover and simmer gently for 15 - 20 minutes or until tender. Transfer the chicken to a plate and cover with foil.
Remove the chicken and strain the liquid. Discard the vegetables and return the liquid to the pan. Add the sliced mushrooms, wine vinegar or lemon juice and simmer for 2 - 3 minutes until tender.
Cook the spaghetti in a large pan of boiling, salted water until al dente. Drain thoroughly.
Blanch the cucumber in boiling water for 10 seconds. Drain and rinse under cold water.
Cut the chicken breasts into bite-size pieces. Boil the stock to reduce by half, then add the chicken, tomatoes, sour cream, cucumber and herbs. Season with salt and pepper to taste.
Transfer the spaghetti to a warmed serving dish and spoon over the chicken. Serve at once.