Preheat oven to 350 F degrees. Lightly spray a 3 qt baking dish or skillet with non-stick cooking spray and set aside.
In a large skillet, cook sausage until browned, about 5 minutes. Remove with a slotted spoon, reserving the drippings in the skillet, to a large mixing bowl with the thawed hashbrown potatoes.
Add onion and bell pepper, cooking until soft, about 3-5 minutes. Remove to large mixing bowl with potato mixture, along with the green chiles.
To same skillet, add butter and melt. Once melted, add flour and whisk to combine, cooking 1 minute. Slowly pour in milk and chicken stock, whisking as you pour. Bring to a boil, then reduce heat to MED-LOW and simmer, stirring often, until thickened to the consistency of a slightly thinned out condensed soup, about 5-10 minutes.
Add 3 cups of cheddar cheese, stirring until melted and combined. Remove from heat and stir in sour cream, salt, pepper and garlic powder.
Add cheese sauce to mixing bowl and stir to combine all ingredients. Transfer mixture to prepared baking dish or skillet.
Sprinkle with remaining 1 cup of cheese, and bake 45-50 minutes, until bubbly and golden brown. If desired, garnish, then serve.