Bring a large pot of salted water to a boil. Add fresh green beans and cook for 3-4 minutes until bright green and tender-crisp. Drain and plunge into ice water to halt cooking, then drain again and set aside.

In a large skillet, melt butter over medium heat. Add sliced mushrooms and sauté for 5-6 minutes until browned and tender. Sprinkle flour over mushrooms, stirring for 1-2 minutes to form a roux, then whisk in milk until the sauce thickens and becomes creamy, about 3-4 minutes. Season with garlic powder, onion powder, salt, and pepper.

Remove skillet from heat and stir in blanched green beans into the mushroom sauce, ensuring all beans are well-coated. Adjust seasoning if needed.

Preheat the oven to 375°F (190°C) and grease a baking dish. Transfer the green bean and mushroom mixture into the prepared dish, spreading evenly. Sprinkle grated parmesan cheese over the top and add a layer of fried shallots.

Bake for 20-30 minutes until the top is golden brown and bubbly.