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)