Preheat the oven to 350°F. Grease an 9×13 inch casserole dish with butter or cooking spray.
In a glass measuring cup (or mixing bowl), whisk together the chicken broth, half and half, cornstarch, parsley, poultry seasoning, salt, and pepper. Set aside.
Heat 1 tablespoon butter in a medium saucepan over medium-high heat. When the butter is melted, add the onions and saute until translucent, about 3 minutes. Stir in the garlic at the end for about 30-60 seconds, until fragrant.
Whisk in the creamy cornstarch mixture to the onions. Bring to a simmer, stirring constantly until thick and creamy, about 2-3 minutes. Remove from the heat, stir in the remaining 4 tablespoons butter, and let cool.
In a large mixing bowl, stir together the thawed hash browns, sour cream, cheese, bacon, and the cooled cream sauce. Place in the casserole dish, smoothing it out evenly. Optional: Sprinkle another ¼ -½ cup cheese on top. (Note: At this point, you can cover and refrigerate for up to 3 days or freeze for later.)
Bake for about 45 minutes, until edges are bubbly, middle is hot, and top is golden.
