Instructions

1. Preheat the oven to 475 F.
2. For the onions: Line a baking sheet with parchment paper.
3. Pour flour in one small bowl.
4. Pour panko, salt, and pepper into a medium bowl.
5. Whisk the egg and milk together in another small bowl.
6. Line a baking sheet with parchment paper.
7. To bread the onion slices, use one hand for dry ingredients and the other hand for wet ingredients. Dip a few onion slices into the flour, then dip them into the egg mixture. Now place them in the panko mixture and coat well. Lay each onion onto the prepared baking sheet. Repeat with the remainder of the onions.
8. Bake onions for 25 minutes or until they are golden brown. Flip the onions twice during the baking time. Set aside.
9. Reduce the oven temperature to 400 F.
10. For the beans: Bring a gallon of water and 1 tablespoon of salt to a boil in a large stockpot.
11. Add the beans and blanch for 5 minutes. Drain the beans and place them into an ice water bath to halt the cooking. Keep them in the ice water bath until they cool, then drain into a colander.
12. For the casserole: In a large skillet over medium-high heat, melt the butter.
13. Add mushrooms, 1 teaspoon salt, and the pepper. Cook, stirring occasionally, for 5 minutes or until the mushrooms begin to release their moisture.
14. Add the garlic, stir, and cook for another 2 minutes.
15. Sprinkle the flour over the mushrooms and stir until combined.
16. Add the chicken broth and simmer for 3 minutes. Decrease the heat to medium-low and add the half-and-half. Stirring occasionally, cook for about 10 minutes or until the mixture is thick.
17. Place green beans in a large bowl and add the mushroom sauce and 1/4 of the onions. Stir to combine.
18. Transfer the green bean mixture to a casserole dish. Top the green beans with the remaining onions.
19. Bake for 10 to 15 minutes or until bubbly.
20. Serve hot.