Use a 6-quart slow cooker. Coat the insert well with cooking spray, then pour in the hash browns (no need to thaw). Add the onion and stir well to combine.
In a very large bowl, combine the cheese, peas, corn, milk, broth, and all seasonings. Stir well to combine. Pour this mixture evenly over the top of the potatoes. Cover and cook on low for 7 hours, or until cooked through. Take the lid off and cook on high for an additional 20-30 minutes to release steam and allow the potatoes on top to brown.