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
• 1/2 teaspoon dried thyme
• 1/4 cup all-purpose flour
• 1/4 cup tomato paste
• 2 pounds ground beef chuck
• 1 cup whole milk
• 1 1/2 cups shredded sharp white cheddar (6 ounces)