Soak the sweetbreads for 1 hour. Blanch them in boiling water for 2 to 3 minutes. Drain and dry the sweetbreads. Cut them in half crosswise. Melt 4 tablespoons butter in a heavy casserole, add the carrots and onions. Add the sweetbreads, bouquet garni, salt and pepper. Cover and simmer for about 1 hour. In the meantime, cook a tablespoon butter with 1/4 cup water, lemon juice and chopped mushrooms. Strain, reserving the mushroom juice. When the sweetbreads are cooked, drain them and cool. Deglaze the casserole with the white port, crÅme fraöche and mushroom juice. Reduce by half, strain, add the mushrooms and keep warm. Give the pastry two more turns and roll it out thinly and cut into 6 rectangles. Place half a sweetbread on each piece of pastry and form into a package. Glaze the pastry with the egg yolk and bake for 5 minutes. Lower the heat to 350F and cook for another 15 minutes.
