• Preheat oven to 375F. Prepare a 9×11 baking dish with nonstick spray.

Make Fried Onions

• In a large bowl place the buttermilk.
• In a separate bowl, combine the cornstarch and season salt.
• Dip the onion slices in the buttermilk. Shake to remove excess and then toss in the cornstarch mixture to coat.
• Heat 2-3 inches of oil in a deep pan to 350F. Place a wire rack or tray lined with paper towels close by.
• In batches, fry the onions in the oil. Stir occasionally with a slotted spoon until golden brown. This will be fast, maybe 2-3 minutes.
• Using the slotted spoon, remove the fried onions and place on the paper towels. Continue with remaining onions until all are fried. Set aside.

Blanche Green Beans

• Make an ice bath in a large bowl with cold water and ice.
• In a large pot, bring water to a boil with 3 teaspoons of kosher salt.
• Add in the green beans and cook for 5 minutes or until crisp tender.
• Remove with tongs and submerge in the ice bath to stop the cooking and preserve their bright green color.
• Cool for 2-3 minutes, drain.

Make Topping

• Combine the 1 tablespoon melted butter and the Panko crumbs. Stir to combine.

Make Casserole

• In a large skillet over medium high heat, melt 4 tablespoons butter. Cook sliced mushrooms, stirring often, until tender and browned (2-3 minutes).
• Season with 1 teaspoon salt, pepper. Stir in garlic and mix to combine.
• Sprinkle the flour over the mushrooms and stir to coat. Slowly pour in the chicken broth, whisking to combine.
• Bring to a simmer and cook for 1-2 minutes.
• Turn heat to medium and whisk in the cream. Stir continuously until the sauce thickens. About 4-5 minutes. Remove from the heat and stir in green beans.
• Spread into the prepared baking dish. Top with Panko mixture. Bake at 375F for 15-20 minutes or until bubbly.
• Remove from the oven and top with fried onions. Serve warm.