Sweet Potato Cottage Pie

Name: Sweet Potato Cottage Pie
Ingredients: 1-1/2 pounds sweet potatoes (about 2 large); 1 tablespoon olive oil; 1 cup chopped onion; 1/4 cup chopped celery; 1 teaspoon finely chopped garlic (1 large clove); 1 pound ground beef, 90% lean; 1 can (15 ounces) tomato sauce; 1 tablespoon Worcestershire sauce; 1-1/2 teaspoons fresh thyme or 1/2 teaspoon dried thyme or herbes de Provence, divided; 1/2 teaspoon salt, divided; 1/2 teaspoon ground black peper; 2 tablespoons butter; 1 tablespoon milk