Instructions

• 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 bowl, panko + salt + pepper into another bowl, and whisk the egg and milk together in a third bowl. Dip onion slices into flour, then egg mixture, and finally into panko. Place onto baking sheet and bake 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. Add green beans and boil for 5 minutes. Transfer to a large bowl of ice water to stop the cooking process, then drain and set aside.
• For the casserole: Melt butter in a large skillet 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 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 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 can be kept in an airtight container in the refrigerator for up to 4 days. Reheat as desired.