Trim the green beans and cut them into half or third, depending on their size.
Blanch the green beans in salted boiling water for 10 minutes, then drain the water well and set them aside.
In a large frying pan, add the butter and allow it to melt.
Add the sliced mushrooms and fry them until golden and all the liquid released has been evaporated.
Add the peeled and chopped garlic, and fry for a further 1 minute.
Sprinkle over the flour and mix well to coat the mushrooms.
Pour over the stock, cream, add the dijon mustard, Worcestershire sauce and parmesan, and mix well to get a creamy sauce.
Season the sauce to taste, then add the green beans.
Mix well to coat the green beans in the creamy sauce.
Transfer everything to an ovenproof dish and bake in the preheated oven at 180 degrees Celsius (350 Fahrenheit) for 20 minutes.
Sprinkle over the crispy fried onions and return the dish to the oven for a further 5 minutes until golden and bubbling hot.