Sweet Potato Cottage Pie

Serves:

• 1 1/2 pounds North Carolina 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
• 1 can (15 ounces) tomato sauce
• 1 tablespoon Worcestershire sauce
• 1 1/2 teaspoon fresh thyme or 1/2 teaspoon dried thyme or herbes de Provence divided
• 1/2 teaspoon salt divided
• 1/4 teaspoon ground black pepper
• 2 tablespoons butter
• 1 tablespoon milk