• Preheat the oven to 350ºF. Spray a 9x11-inch casserole dish and a standard-size baking sheet with nonstick cooking spray. Set both aside.

For the Onions

• In a large bowl, whisk the egg and water together. In a separate smaller bowl, mix the breadcrumbs, parmesan, salt, and pepper together.
• Slice 2 large onions - we used setting #4 on the mandoline (a sharp knife works, too). Add the sliced onions to the bowl with the egg mixture. Stir to coat the onions.
• Add the breadcrumb mixture to the onions. Gently fold everything together until the onions are well coated.
• Spread the onions onto the prepared baking sheet and set aside.

For the Green Bean Casserole

• Place the rinsed and trimmed green beans into the prepared casserole dish. Set aside.
• Heat the olive oil in a large saucepan set over medium/high heat. When the oil is fragrant, add the mushrooms and 1/8 teaspoon of salt. Sauté for 4-6 minutes. Remove the mushrooms from the heat and transfer to a separate bowl. Set aside.
• In the same saucepan, prepare the roux. Melt the butter over medium/high heat. Add the flour to butter and mix until a thick paste forms.
• Slowly pour in the milk while continually whisking the milk, butter, and flour together. Whisk until mixture is thick. Remove the saucepan from the heat.
• Add the broth, cooked mushrooms, and the remaining 1/8 teaspoon of salt to the roux. Mix well.
  
• Pour the roux over the green beans and toss (directly in the baking dish) so the beans are coated.
  
• Place both the green bean casserole and onions in the oven at the same time.
• Bake for 20 minutes. Toss the onions and bake for an additional 20 more minutes.
• After 40 minutes, remove the casserole from oven. For extra crispy onions, turn the oven to high broil and broil the onions for 3-5 minutes.*
• Once onions are crunchy, use a spatula to gently transfer onions to top the green bean casserole. Serve warm.