Thaw the potatoes: Place frozen diced hashbrowns in a medium bowl to thaw. To speed up thawing, microwave for 1 minute or refrigerate overnight. Pat dry before using.
Cook the onions: Heat olive oil in a 10-inch skillet over medium heat. Add onions, salt, and pepper, and cook, stirring frequently, until onions are translucent, about 2 minutes.
Add garlic and cook for 30 seconds, stirring frequently.
Make the sauce: Pour in chicken broth, increase heat to medium-high, and bring to a simmer, about 30 seconds.
Stir in heavy cream and let the sauce thicken, stirring frequently, for about 2 minutes.
Remove from heat and mix in sour cream.
Assemble the casserole: Stir the thawed potatoes into the sauce until fully coated.
Mix in the cheese: Add ⅓ cup of shredded cheese and stir to combine.
Transfer to a baking dish: Spoon the mixture into a lightly buttered 5x5-inch baking dish.
Make the topping: In a small bowl, mix breadcrumbs, the remaining ⅓ cup of cheese, and melted butter.
Top the casserole: Sprinkle the breadcrumb mixture evenly over the potatoes. Place the dish on a rimmed baking sheet to catch any spills.
Bake for 30 minutes until hot and golden brown.
Let it rest for 5 minutes before serving.
Notes
Read the recipe first: This dish comes together fast, so review the recipe and prep your ingredients before starting.
Topping options: We use a breadcrumb topping, but you can swap it for ½ cup of crushed Corn Flakes or 8 crushed buttery crackers (like Ritz or Club).
If doubling this recipe, double the ingredient amounts and bake in either two 5x5-inch baking dishes or one 6x8-inch baking dish.