Cheddar Topped Shepherds Pie

Name: Cheddar-Topped Shepherd's Pie
Ingredients: 2 pounds baking potatoes (about 4), peeled and thinly sliced; Coarse salt and ground pepper; 1 tablespoon vegetable oil, such as safflower; 6 medium carrots, halved lengthwise, quartered if large, and thinly sliced; 6 celery stalks, thinly sliced; 1 large onion, chopped; ½ teaspoon dried thyme; ¼ cup all-purpose flour; ¼ cup tomato paste; 2 pounds ground beef chuck; 1 cup whole milk; 1 ½ cups shredded sharp white cheddar (6 ounces)