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. In bowls, pour flour, panko with salt and pepper, and whisk the egg and milk together. Bread the onion slices and place them on the baking sheet. Bake until golden brown, about 20 minutes, flipping twice.
• Reduce oven temperature to 400°F (204°C).
• Blanch the green beans: Bring water and 1 tablespoon of salt to a boil. Boil the green beans for 5 minutes, then transfer to ice water. Drain and set aside.
• For the casserole: Melt butter in a 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, stir until combined, then add broth and simmer for 3 minutes. Reduce heat and stir in half-and-half, cooking until thick, about 10 minutes.
• Remove from heat and add a quarter of the onions and all 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.
• Leftovers keep well in an airtight container in the refrigerator for up to 4 days. Reheat as desired.