Potatoes:
3 tablespoons butter
1 large yellow onion, chopped
1/4 cup all-purpose flour
1 1/2 cups low-sodium chicken broth
1 cup milk
1 ½ teaspoons salt
1/2 teaspoon pepper
1/2 teaspoon dried thyme
2 cups shredded sharp cheddar cheese (8 ounces)
26-ounce bag frozen shredded hash browns
1/2 cup light sour cream

Topping:
3 cups cornflakes, lightly crushed
2 tablespoons butter, melted