For the veal, place the cubes of meat into a large saucepan, cover with cold water and bring to the boil.
When the water is boiling, reduce the heat until the water is simmering gently, then simmer for 30 minutes, skimming off any froth that collects of the surface of the liquid at intervals.
Add the vegetables, bouquet garni and a large pinch of salt, or to taste. Continue to simmer for a further 1 hour 20 minutes, adding more water as necessary to cover the veal and vegetables. (NB: The pan contents should be covered by the water at all times.)
While the meat is simmering, carefully decant 500ml/18floz of the cooking liquid.
Meanwhile, for the pilau rice, heat the butter and olive oil in a frying pan over a medium heat. Add the onion and garlic and fry for 2-3 minutes, or until softened.
Add the rice and stir well to coat.
Pour in the stock, stir well, then bring the mixture to the boil.
Reduce the heat until the liquid is simmering. Cover the pan and continue to simmer for 15-18 minutes until the rice has absorbed the stock and is tender. Season, to taste, with salt and freshly ground black pepper.
Meanwhile, for the onions and mushrooms, place the cocktail onions into a separate pan and pour over the decanted cooking liquid. Bring the cooking liquid to the boil, then reduce the heat until the liquid is simmering.
Continue to simmer the onions for 10-15 minutes, or until tender. Add the mushrooms, cover the pan with a lid and continue to simmer for a further 10 minutes, or until the vegetables are tender.
Drain the onions and mushrooms, collecting the cooking liquid in the pan with the veal in it. Keep the drained onions and mushrooms warm.
When the veal pieces are tender, remove them from the pan and set aside. Keep warm.
For the sauce, strain the cooking liquid from the onions and mushrooms into a clean saucepan (discard the vegetables).
Bring the cooking liquid to a boil and continue to boil for 12-15 minutes, or until the volume of liquid has reduced by half.
Stir in the double cream, then return the mixture to the boil for a further 4-5 minutes.
Meanwhile, in a bowl, mix together the egg yolks and crème fraîche until well combined.
Remove the pan from the heat, then stir in the egg yolk and crème fraîche mixture. Stir continuously until the mixture is completely incorporated and the sauce has thickened slightly. Season, to taste, with salt and freshly ground black pepper, then strain the mixture through a fine sieve.
To serve, spoon some of the veal, onions and mushrooms into the centre of each of six to eight serving plates. Drizzle over the sauce.