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 slices into flour, then egg mixture, and finally into 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: 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. Drain the beans and immediately transfer them into the bowl of ice water. 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 pepper. Cook until the 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. Cook until thick—about 10 minutes.
• Remove from heat, add 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.