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 using one hand for dry and one hand for wet. Dip slices into the flour, then the egg mixture, and finally coat in panko. Place onto baking sheet. Bake onions until golden brown, about 20 minutes, flipping twice. Set aside.
• Reduce oven temperature to 400°F (204°C).
• Blanch the green beans: Boil a gallon of water with 1 tablespoon of salt. Add the green beans and boil for 5 minutes. Transfer them to a bowl of ice water to stop cooking. Drain and set aside.
• For the casserole: In a large ovenproof skillet, melt 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 to medium-low, add half-and-half, and cook until thickened, about 10 minutes.
• Remove from heat, add ¼ of the onions and green beans. Stir to combine. If not using an oven-safe skillet, transfer to a greased casserole dish. Top with remaining onions and bake until bubbling, about 15 minutes.
• Remove from oven and serve warm.
• Leftovers keep well in an airtight container in the refrigerator for up to 4 days. Reheat as desired.