Wash and trim the fresh green beans. Blanch them in salted boiling water for 3-4 minutes, then transfer to an ice bath. Drain and set aside.

Slice the shallots thinly and fry in oil over medium heat for 3-5 minutes until golden brown. Drain on paper towels and sprinkle with salt.

In the same skillet, sauté diced mushrooms and minced garlic for 5-7 minutes. Sprinkle with flour, whisk in broth gradually, followed by heavy cream, and cook until thickened, about 5-8 minutes. Stir in Parmesan, salt, and pepper.

Combine blanched green beans with the creamy mushroom sauce in a mixing bowl, ensuring beans are well coated.

Preheat the oven to 375°F (190°C). Transfer the mixture to a greased baking dish, top with fried shallots, and bake for 20-30 minutes until bubbly.

Let the casserole cool for 5 minutes before serving.