Instructions

1. Preheat the oven to 350ºF and spray a 9×11-inch casserole dish and a standard-size baking sheet with cooking spray. Set aside.
2.  First, prepare onions. Whisk together one egg and 2 tablespoons of water in a large bowl. In a separate bowl, mix together bread crumbs, parmesan, salt, and pepper.
3. Then, slice 2 large onions. Add sliced onions to the egg mixture. Stir mixture until all of the onions are coated with egg.
4. Pour the breadcrumb mixture on top of the onions and gently fold everything together.
5. Spread onions out onto a baking sheet and set aside.
6. Next, prepare green bean casserole. Heat a large saucepan over medium/high heat. Add olive oil. When olive oil is fragrant, add mushrooms and 1/8 teaspoon of salt and saute for 4-6 minutes. Remove from heat and transfer mushrooms into a separate bowl. Set aside.
7. In the same saucepan, prepare the roux. Melt butter over medium/high heat. Add flour to butter and mix until a paste is created.
8. Slowly add milk while continually whisking the milk, butter, and flour together. Whisk until mixture is thick. Remove from heat.
9. Add broth, cooked mushrooms, and the last 1/8 teaspoon of salt to the roux and mix well.
10. Place uncooked green beans into the casserole dish and then pour the roux over green beans and toss so that the beans are coated.
11. Place both the green bean casserole and onions in the oven at the same time.
12. Bake at 350ºF for 20 minutes. Then toss the onions and bake for an additional 20 more minutes.
13. Remove casserole from oven and then turn the oven to high broil. Broil the onions for 3-5 minutes for extra crispy onions*.
14. Once onions are crunchy, use a spatula to gently transfer onions on top of the green bean casserole.