01 - Preheat oven to 375°F. Blanch green beans in boiling water for 3 to 4 minutes, then drain and rinse under cold water.
02 - In a skillet, melt butter over medium heat. Sauté mushrooms and chopped onion until softened.
03 - Sprinkle flour over the vegetables and cook for 1 minute, stirring constantly. Gradually whisk in milk and vegetable broth. Simmer until the sauce thickens.
04 - In a baking dish, combine blanched green beans with the mushroom sauce. Mix thoroughly to ensure even coating.
05 - Sprinkle crispy fried onions evenly over the top and bake for 20 to 25 minutes, until the casserole is bubbly and golden on top.