Prepare the Ingredients. Trim the ends of the green beans and slice the mushrooms, onions, and garlic.
Sauté Vegetables. In a skillet, sauté the mushrooms, onions, and garlic in butter until softened.
Create the Sauce. Sprinkle flour over the sautéed vegetables, stir to combine, and cook briefly. Slowly add chicken broth and heavy cream, stirring constantly until the mixture thickens.
Layer in the Crockpot. Place the trimmed green beans in the crockpot and pour the sauce over them. Mix well to ensure the green beans are evenly coated.
Cook Slowly. Cover and cook on low for 4-6 hours or until the green beans are tender.
Top and Serve. Before serving, sprinkle fried onions on top for a crispy finish.
Garnish and Enjoy. Transfer the casserole to a serving dish, garnish with additional fried onions if desired, and serve!