Preheat the oven to 375 degrees F. Grease a 9 x 13-inch pan and set aside.

Bring a large pot of salted water to a boil. Add the green beans and blanch them for 3 to 4 minutes. Drain, and immediately place them into a large bowl filled with ice water to stop the cooking process. Drain again. Place the green beans on a clean kitchen towel or plate lined with a paper towel. Set aside.

In a large skillet, melt 2 tablespoons of butter. Add the mushrooms, 1/2 teaspoon salt, and the pepper. Cook for about five minutes stirring occasionally. Stir in the garlic and cook for another two minutes.

Sprinkle the flour on top of the mushrooms, stir until well combined. Add the vegetable broth and simmer for 2 to 3 more minutes. Reduce the heat to medium low and add the heavy cream. Stirring occasionally, cook until the sauce has thickened, about 5 to 10 minutes.

Remove from heat and add the green beans and 1 cup of fried onions. Stir until sauce and beans are well combined. Pour into the prepared baking dish.

In a medium skillet, melt remaining tablespoon of butter. Add the panko breadcrumbs and cook until the breadcrumbs are toasted, 3 to 4 minutes. Add salt and pepper to taste.

Top with the remaining onions, toasted breadcrumbs, and parmesan cheese. Bake uncovered for 20 minutes or until bubbly.

Serve and enjoy!