How to Make It
Preheat the oven to 350°. In a large enameled cast-iron casserole, cover the veal with cold water and bring to a boil over high heat, skimming as necessary. Drain the meat in a colander. Return the meat to the casserole and add the carrots, onion, bouquet garni, stock and wine. Cover and bring to a boil over high heat, then cook the veal in the oven for about 2 1/2 hours, or until very tender.
Meanwhile, in a large nonreactive skillet, melt the butter over moderately high heat. Add the leeks and mushrooms and cook, stirring, for about 1 minute. Add 1/2 cup of water, the lemon juice and 1 teaspoon of salt and reduce the heat to low. Cook until the leeks and mushrooms are tender, about 10 minutes.
Transfer the stew to a colander set over a bowl. Discard the spice bag. Transfer the meat and vegetables to a large shallow bowl, cover and keep warm. Pour the broth back into the casserole and boil over high heat until reduced to 2 cups, about 15 minutes. Add the peas and cook until tender, about 2 minutes. Using a slotted spoon, remove the peas from the broth and add them to the bowl with the veal.
In a small bowl, whisk the egg yolks and cream. Whisk in 1/2 cup of the hot broth. Add the egg-yolk mixture to the casserole and whisk constantly over moderately low heat until the sauce thickens, about 8 minutes; do not let the sauce boil. Remove from the heat and continue whisking for a minute or two to stop the cooking. Stir in the tarragon and season with salt and pepper. Pour the sauce over the veal and vegetables in the bowl and serve.