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In a small saucepan over medium heat, melt the butter.
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Add in the chopped mushrooms and onion and saute until tender. Add in the garlic powder. Briskly whisk in the flour until a thick dough forms. Cook for about 1 minute.
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Very slowly add in the milk whisking constantly until thickened.
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Reduce the heat to low and allow to simmer and thicken. Remove from heat and stir in the sour cream.
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Spray a large slow cooker (I used a 6-quart crockpot) with nonstick spray.
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Pour in the creamy base. Top with the whole package of hashbrowns and the shredded cheddar cheese (I like to use freshly shredded as the packaged tends to get a bit greasy). If you like an extra cheesy casserole use a full cup, otherwise use just 3/4 a cup.
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Add salt and pepper and stir everything together.
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Cover and cook on high for 2-3 hours. Remove the lid and allow to cook another 30 minutes to an hour to get crispier on top. Just watch these times carefully depending your slow cooker -- if you have a really hot one it may be done sooner and a cooler one will take longer. Each crockpot is so different!
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Top with toppings of your choice such as cooked and chopped bacon, chopped parsley, additional seasonings, etc.