Preheat oven to 375 degrees F. In a covered medium saucepan cook sweet potatoes and garlic in enough boiling lightly salted water to cover for 15 to 20 minutes or until tender. Drain well. Mash with a potato masher or beat with an electric mixer on low speed just until smooth. Gradually add milk and salt, mashing or beating to make potato mixture light and fluffy. Cover and keep warm.
Meanwhile, in a large skillet cook turkey and onion over medium heat until meat is brown, using a wooden spoon to break up meat as it cooks. Drain off fat. Stir zucchini, carrots, corn, and the water into meat mixture in skillet. Bring to boiling; reduce heat. Simmer, covered, for 5 to 10 minutes or until vegetables are tender.
Stir tomato sauce, Worcestershire sauce, snipped sage, and pepper into meat-vegetable mixture; heat through. Divide among four 10-ounce ramekins or individual casseroles, spreading evenly. Pipe or spoon mashed sweet potato mixture in mounds onto meat-vegetable mixture.
Bake for 20 to 25 minutes or until heated through. If desired, garnish with fresh sage leaves.