Place veal cubes in a saucepan, cover with cold water, and bring to boil.
Gently simmer for 30 minutes while skimming any froth from the surface.
Add vegetables, bouquet garni, and salt; simmer for another 80 minutes.
Decant 500ml of the cooking liquid while meat continues to simmer.
Heat butter and oil in a pan and fry onion and garlic until softened.
Add rice and stir well to coat with fat.
Pour in 450ml stock and bring the mixture to a boil.
Cover and simmer rice for 15-18 minutes until liquid is absorbed and rice is tender.
Place cocktail onions in a separate pan with 500ml decanted liquid and bring to boil.
Simmer onions for 10-15 minutes, add mushrooms, cover, and simmer for 10 more minutes.
Drain onions and mushrooms, returning the liquid to the veal pan and keeping vegetables warm.
Remove tender veal pieces from the liquid and set aside to keep warm.
Strain the cooking liquid into a clean saucepan and discard the aromatics.
Boil the liquid for 12-15 minutes until reduced by half.
Stir in double cream and boil for a further 4-5 minutes.
Whisk together egg yolks and crème fraîche in a separate bowl.
Remove sauce from heat, whisk in egg mixture until thickened, then strain.
Plate the veal, onions, and mushrooms and drizzle with the finished sauce.