This savory and hearty pie is full of flavorful sweet potatoes.
Preheat oven to 350 degrees. Oil a 6- to 8-cup shallow baking dish.
Pierce sweet potato skins; microwave until tender, about 4 to 6 minutes; cool. In large skillet, heat oil over medium heat. Add onion and celery; cook and stir until tender, about 5 minutes. Add garlic; cook and stir 1 minute. Add beef; cook and stir until it just begins to brown. Stir in tomato sauce, Worcestershire, 1 teaspoon of the thyme, and 1/4 teaspoon of the salt and pepper; bring to a boil.
Transfer to baking dish; spread evenly. Scoop sweet potato from skins into medium bowl; add butter, milk, remaining 1/2 teaspoon thyme and 1/4 teaspoon each salt and pepper. With electric beater, beat until smooth; spread over meat in baking dish. Bake until bubbly, about 20 to 25 minutes.