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 plus salt and pepper into a medium bowl, and whisk the egg and milk together in another small bowl. Dip the onion slices first into flour, then into the egg mixture, and finally into the panko. Place onto baking sheet. Bake until golden brown, 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 the green beans and boil for 5 minutes. Transfer to a bowl of ice water to stop cooking. Drain and set aside.
• For the casserole: In a large ovenproof skillet, melt the butter over medium-high heat. Add 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 and add half-and-half, cooking until thickened, about 10 minutes.
• Remove from heat and add about a quarter of the onions and all green beans, stirring to combine. Transfer 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 keep well in the refrigerator for up to 4 days. Reheat as desired.