Soak the mushrooms in warm water for 15 minutes. Drain and rinse them at least twice. Soak the sweetbread in cold water with a little vinegar for 2 hours. Cut away and discard any discolored parts. Place the sweetbread in a medium saucepan with water to cover and bring to a boil. Reduce heat and simmer for 5-8 minutes. Cool the sweetbread in the cooking liquid. In a frying pan melt the butter and cook the shallots until softened, about 5 minutes. Chop the sweetbread and half the mushrooms and stir them into the shallots along with the truffle. Season with salt and pepper. Stuff the thighs with this mixture and truss or secure with a poultry pin. Brown the thighs in the butter. Remove the thighs from the frying pan and discard the cooking fat. Place the thighs in an ovenproof casserole or Dutch oven and pour in the white wine and chicken stock. Bring to a boil. Reduce heat and simmer 20-30 minutes. Remove the thighs witha slotted spoon and keep warm. Strain the sauce and reserve it. In a medium saucepan melt 2 tablespoons of the butter. Whisk in 2 tablespoons of flour and cook for 1-2 minutes. Stir in the cooking liquid and boil hard until thickened, stirring occsionally. Transfer the thighs to the sauce, along with the remaining chopped mushrooms. Simmer for 5 minutes. In a small bowl whisk together the egg yolks and crÅme fraöche. Stir yolk mixture into sauce and heat through. Place the thighs on a serving platter and spoon over the sauce. Serve immediately.
