COMBINE THE VEAL and 4 cups broth in a 5-quart heavy pot over high heat on top of the stove. Cover and bring to a boil. Remove any scum that comes to the top of the pot, then add the remaining broth, the onion, celery, carrot, salt, pepper, nutmeg and lemon. Reduce heat to low and let simmer for 1 1/4 hours, or until meat is just tender. Meanwhile, melt the butter in a pot, over low heat, and whisk in the flour. Cook, stirring, 2 minutes and remove from heat. Set aside until the meat is cooked. When the meat is done, remove it from the liquid with a slotted spoon. Pick off and discard any vegetables that cling to the veal. Place the butter-flour mixture over medium-low heat on the stove. Strain the cooking liquid into it, whisking, and add the sherry and cream. Discard vegetables in the strainer. Cook, stirring, until mixture thickens, about 3 to 5 minutes. Replace the meat in the pot and add the mushrooms and crayfish. Pour in the sauce, place the pot, uncovered, over medium heat and cook 10 minutes. To serve, pour into a serving dish.