Preparation Steps

• Preheat the oven to 450°F (232°C). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
• For the onions: Thinly slice the onion and separate the slices. Pour flour in one bowl, panko + salt + pepper into a medium bowl, and whisk the egg and milk in another bowl. Dip onion slices first into flour, then egg mixture, then panko. Place on baking sheet and bake for about 20 minutes, flipping twice.
• Reduce oven temperature to 400°F (204°C).
• Blanch the green beans: Bring a gallon of water and 1 Tablespoon of salt to a boil. Add green beans and boil for 5 minutes. Transfer them into a bowl of ice water to stop cooking, then drain and set aside.
• For the casserole: Melt butter in a large ovenproof skillet. Add mushrooms, 1 teaspoon salt, and pepper; cook for 5 minutes. Add garlic, cook for 2 minutes. Sprinkle with flour and stir. Add broth and simmer for 3 minutes. Reduce heat, add half-and-half and cook until thick, about 10 minutes.
• Remove from heat and add about a quarter of the onions and all of the green beans. Stir to combine. Top with remaining onions and bake until bubbling, about 15 minutes. Serve warm.
• Leftovers keep well in an airtight container in the refrigerator for up to 4 days. Reheat as desired.