Begin by blanching the green beans in boiling salted water for about 3-4 minutes until bright green and tender-crisp. Transfer to ice water to halt cooking, drain, and set aside.

In a large skillet, melt the butter over medium heat, then add the sliced mushrooms. Sauté for 5-7 minutes until browned and fragrant.

Sprinkle in the flour, stirring until combined, and cook for an additional minute to form a roux. Gradually whisk in the milk until the mixture thickens, then season with garlic powder, onion powder, salt, and pepper.

Add the blanched green beans to the skillet, stirring well to coat each bean in the sauce. Adjust seasoning to taste.

Pour the mixture into a greased 2-quart baking dish, sprinkle with Parmesan cheese, and top with fried shallots.

Preheat the oven to 375°F and bake for 20-30 minutes until golden brown and bubbly. Allow to cool slightly before serving.