1. Heat the oil in a large skillet over medium-low heat. Add sausage and sauté, breaking up the meat with a fork, until the meat begins to lose its pink color. Add the leeks and cook until the leeks are tender and the meat begins to brown, 15 to 20 minutes.
2. Reduce heat to low and stir in the chevre, breaking it into small pieces. Cook until the chevre melts. Season with thyme and salt and pepper to taste.
3. Spoon the sausage mixture into the pocket of the veal. Secure with small skewers or tie with kitchen string.
4. Preheat oven to 3750 F.
5. Line a roasting pan large enough to hold the veal with aluminum foil. Scatter the onion, carrots, and celery over the foil and place the stuffed breast of veal on the vegetables.
6. Mince the parsley, garlic, and lemon zest in a food processor fitted with a steel blade or by hand. Pat this mixture over the top of the veal.
7. Pour 1 ½ cups of wine around the veal. Fold the edges of the foil around the veal and cover the top with more foil. Bake 1 ½ hours.
8. Remove the top foil and pour in the remaining ½ cup wine if the vegetables are too dry. Bake uncovered until the top is nicely browned, about 1 more hour.
9. Remove from the oven and let stand 10 to 15 minutes. Cut the veal into pieces between the rib bones and serve immediately.