Crispy Onion Topping:

1. Preheat oven to 475 degrees.
2. Thinly slice two onions and separate each individual onion piece. In a large bowl, beat egg, add onions and toss to coat. After onions are tossed in egg, add bread crumbs, flour and ½ tsp salt. Toss to combine. Spread onion mixture on a greased or parchment paper lined baking sheet. Bake for 10 minutes, toss and bake an additional 5 minutes. Watch to make sure onions don’t burn. Reduce oven temperature to 350 degrees for green bean casserole.

Sauce: (Can Prepare the Night Before)

1. In a medium-size pot on medium heat add EVOO, garlic and mushrooms. Cook until mushrooms are brown and tender, about 4-5 minutes.
2. In a separate bowl, slowly add flour to milk and whisk to eliminate any lumps.
3. Reduce heat to low. Pour in the milk/flour mixture and stir until combined. Add Greek yogurt, water, cheese, salt and pepper. Stir until combined.
4. Bring sauce to a medium boil, stirring every minute or two until sauce thickens. Once sauce thickens, about 3-6 minutes, remove from heat.
5. If preparing the night before, allow sauce to cool completely and store in the fridge overnight in an airtight container. In the morning, remove from the fridge and re-warm sauce over low heat until it reaches a creamy consistency again.

Combine Casserole:

1. Grease a 2-quart casserole dish. Add green beans to sauce, mixing until green beans are evenly coated. Transfer casserole to prepared baking dish.
2. Loosely cover with foil and bake at 350 degrees for 25 minutes, remove foil and top with onion topping. Bake (uncovered) until center is warm, about 5-10 more minutes. Remove from oven and serve immediately.