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.