Overnight Hashed Brown Potatoe Casserole

• 1 onion
• 1/2 hr covered
• 1/2 hr
• 575 gr hashed brown potatoes (frozen)
• 475 ml sour cream
• 180 ml milk be used in place of one of the sour cream
• 180 ml milk
• 120 gr chedder cheese
• 110 gr butter
• 1 package mushroom soup