Adjust the oven's rack to the middle position and preheat the oven to 350℉/177℃. Lightly grease a 9"x13" casserole dish with cooking spray.
Place a cast-iron skillet or pan over medium-low heat, add bacon and cook until golden and crisp, 4-6 minutes. Transfer the bacon bits to a paper-towel-lined plate and set aside. Pour off all except 1 tablespoon of the bacon drippings.
Add the chopped onions, garlic, and green bell pepper to the pan and cook until the onions are translucent and softened (3-4 minutes). Remove the skillet from the stove and set it aside.
In a large bowl, add thawed hash brown potatoes, condensed cream of chicken soup, sour cream, melted butter, shredded cheese (reserve ¼ cup for the topping), cooked onion and pepper mixture, half of the bacon bits, salt, and pepper. Stir using a spatula until thoroughly combined.
Pour the hash brown potato mixture into your prepared baking pan and press down slightly, creating an even layer. Cover it with foil, then bake for 45-50 minutes, or until it bubbles. The internal temperature should register at least 165-175°F/75-80℃.
Remove the foil, top with the reserved shredded cheese, and bake for 15 more minutes, or until golden brown and bubbly.
Remove the hash brown casserole from the oven and let it rest for 5-10 minutes before serving. Garnish with chopped green onions and the remaining bacon. Enjoy!