Bring a large pot of water to a boil. Salt it, and blanch the onions 1 minute. Remove the onions, and set aside. In the same pot, blanch the meat 4 minutes. Drain, and rinse under cold running water.
Put the veal in a pot and cover with all but 1 cup chicken stock. (If you need more liquid, add water.) Drop in the bouquet garni, carrot, and clove-studded onion. Bring the mixture to a boil, reduce the heat, and simmer very gently, uncovered, 1 to 1 1/2 hours.
While the meat cooks, peel the blanched onions. Melt 2 tablespoons of the butter in a pan and gently fry the onions together with the mushrooms until golden, about 5 minutes. Pour over the remaining cup of stock, and cook until tender and all the liquid has evaporated, about 10 minutes. Set aside.
When the meat is very tender, but not falling apart, remove it from the heat. Discard the carrot, onion, and bouquet garni. Strain the liquid into a saucepan. You should have about 2 cups: if you have more, simply boil to reduce to the right volume. Set the meat aside with the mushrooms and onions, and keep warm.
Melt the remaining 3 tablespoons butter in a saucepan. Whisk in the flour, and cook 1 minute. Pour over the cooking liquid, and simmer to thicken, about 10 minutes, skimming off any film that rises to the surface. Add the creme fraiche, season with salt and pepper, and pour over the meat. Gently reheat, and transfer to a dish, scatter of the parsley, and serve.