Casserole Preparation

• Preheat the oven to 450°F (232°C). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
• 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. Use one hand for dry and one for wet to bread the onion slices. First, dip a few slices into the flour. Then into the egg mixture. Finally, dunk into the panko and coat well. Place onto baking sheet. Repeat with the rest of the onions. Bake until golden brown, about 20 minutes, flipping twice.
• Reduce oven temperature to 400°F (204°C).
• Bring a gallon of water and 1 Tablespoon of salt to a boil in a large saucepan. Add green beans and boil for 5 minutes. Fill a large bowl with ice water and set near the sink. Drain beans and immediately transfer to the bowl of ice water to stop cooking. Drain again, and set aside.
• Over medium-high heat, melt butter in a large ovenproof skillet. Add mushrooms, 1 teaspoon salt, and pepper. Cook, stirring occasionally, until mushrooms begin to release moisture, about 5 minutes. Add garlic, stir, 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. Stirring occasionally, 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 sauce and beans. Top with remaining onions and bake until bubbling, about 15 minutes. Serve warm.