In a saucepan, melt butter and sauté the mushrooms until soft. Stir in flour and cook for 1 minute.
Gradually add the chicken broth and cream, whisking until smooth. Cook until thickened. Add chopped thyme and rosemary, and season with salt and pepper.
In a large mixing bowl, combine the blanched green beans with the mushroom sauce.
Transfer to a baking dish and top with Parmesan cheese, breadcrumbs, and fried onions.
Bake for 25-30 minutes until the top is golden brown and crispy. Serve hot.
Notes
For best results, use fresh green beans and a homemade mushroom sauce. Feel free to adjust the herbs and seasonings to your taste.