Fill a large saucepan with water and bring to a boil over high heat.
Add beans to boiling water and let boil for 5 minutes.
Drain beans and add mushrooms then set aside.
Melt butter in a large skillet over medium heat.
Add onion and parsley and cook for a few minutes until onion is tender.
Whisk in flour, salt and pepper until well combined then gradually whisk in milk, stirring constantly to avoid lumps.
Whisk in sour cream and continue stirring over medium heat until sauce begins to bubble.
Fold in green beans and mushrooms until evenly coated.
Add half the cheese and fried onions and mix until well combined.
Pour green bean mixture into a 9 x 13 inch baking dish sprayed with nonstick cooking spray.
Sprinkle remaining cheese and onions on top.
Bake for 15-20 minutes or until cheese is melted.
Notes
This Green Bean Casserole without Condensed Soup could definitely be made ahead of time to save you time on the big day. Follow the recipe but skip the cooking until the day of.Store in the refrigerator for up to 24 hours ahead of time and in an airtight container or in the baking pan tightly covered in plastic wrap.