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. To bread the onion slices, dip a few slices into the flour, then into the egg mixture, and finally into the panko. Place onto baking sheet. Repeat with the rest of the onions. Bake onions until golden brown, about 20 minutes, flipping them twice. Set them 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 set near the sink. Drain the beans and immediately transfer them into the bowl of ice water to stop the cooking process. Drain again and set aside.
• For the casserole: Over medium-high heat, melt the butter in a large ovenproof skillet. Add the mushrooms, 1 teaspoon salt, and the pepper. Cook until mushrooms release moisture—about 5 minutes. Add garlic, cook for another 2 minutes. Sprinkle flour on top and stir until combined. Add broth and simmer for 3 minutes. Reduce heat and add half-and-half, cooking until mixture thickens—about 10 minutes.
• Remove from heat, 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.