Preparation

• 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 small bowl, panko + salt + pepper into a medium bowl, and whisk the egg and milk together in another small bowl. Bread the onion slices by dipping them in flour, then egg mixture, and finally coat them in panko. Place onto baking sheet. Bake until golden brown, about 20 minutes, flipping twice during this time. Set aside.
• 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 in a large saucepan. Add the green beans and boil for 5 minutes. Fill a large bowl with ice water and transfer the beans immediately to stop the cooking process. Drain again and set aside.
• For the casserole: In a large ovenproof skillet, melt the butter over medium-high heat. Add the mushrooms, 1 teaspoon salt, and pepper. Cook until mushrooms release moisture, about 5 minutes. Add garlic and cook for another 2 minutes. Sprinkle flour on top and stir until combined. Add chicken broth and simmer for 3 minutes. Reduce heat to medium-low and add half-and-half. Cook until thickened, about 10 minutes.
• Remove from heat and add a quarter of the onions and all of the green beans. Stir to combine. Transfer mixture to a greased casserole dish if not using an oven-safe skillet. Top with remaining onions and bake until bubbling, about 15 minutes. Serve warm.
• Leftovers can be kept in an airtight container in the refrigerator for up to 4 days. Reheat as desired.