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1
Cut meat away from veal breastbone, reserving bone, and cut meat into 2-inch pieces.
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2
Bring veal breast and shoulder, veal bone, and water to a boil over moderate heat in a 7- to 8-quart heavy pot, skimming froth.
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3
While water is heating, wrap parsley, thyme, bay leaf, and peppercorns in a small square of cheesecloth and tie into a bundle to make a bouquet.
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4
Add bouquet garni and onions to pot and simmer, uncovered, until veal is tender, 1 1/4 to 1 1/2 hours.
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5
Preheat oven to 300F.
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6
Transfer veal with a slotted spoon to a heatproof serving dish and keep warm in oven, covered with foil.
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7
Discard veal bone, onions, and bouquet garni, then pour stock through a fine sieve into a large bowl.
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8
Return stock to cleaned pot, add carrots and leek, and simmer until tender, 10 to 12 minutes.
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9
Transfer vegetables to serving dish.
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10
Boil stock until reduced to about 2 1/2 cups, about 10 minutes.
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11
While stock is reducing, heat butter in a 10-inch heavy skillet over moderate heat until foam subsides, then cook mushrooms, stirring, until just tender, 6 to 8 minutes.
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12
Transfer to serving dish and season veal and vegetables with salt and pepper.
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13
Keep warm in oven.
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14
Melt butter in a 2- to 3-quart heavy saucepan over moderately low heat, then stir in flour.
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15
Cook roux, stirring, 3 minutes (do not let brown).
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16
Whisk in reduced stock and simmer, uncovered, whisking occasionally, 15 minutes.
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17
Whisk together yolks and creme fraiche in a small bowl, then whisk in 1 cup sauce.
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18
Whisk yolk mixture into remaining sauce with lemon juice, then cook over moderately low heat (do not let boil), stirring constantly, until it reaches 160F on an instant-read thermometer and coats back of a wooden spoon.
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19
Season sauce with salt and pepper and pour over veal and vegetables.
