Caramelize the onions.
Blanch the green beans.
Sauté the mushrooms in butter, then stir in flour and milk to create a creamy mushroom sauce.
Layer the green beans, mushroom sauce, caramelized onions, and Gruyère in a casserole dish.
Sprinkle with the crouton topping and more Gruyère.
Bake at 375°F.
