Step 1: Prepare the Topping

1. In a medium bowl, combine the crispy fried onions, Panko breadcrumbs, and grated Parmesan cheese.

2. Pour the melted butter over the mixture and toss with a fork until everything is evenly moistened.

3. Set the topping aside. This can be done first to get it out of the way.

Step 2: Blanch the Green Beans

1. Bring a large pot of salted water to a rolling boil.

2. Prepare a large bowl of ice water and set it next to the stove. This is your ice bath.

3. Add the trimmed fresh green beans to the boiling water and cook for 4-5 minutes. They should be bright green and just barely tender-crisp.

4. Using a slotted spoon or spider strainer, immediately transfer the blanched green beans from the boiling water into the ice bath. This shocks the beans, stops the cooking process instantly, and locks in their vibrant color and crunch.

5. Let the beans cool in the ice bath for a few minutes, then drain them thoroughly and pat them dry. Set aside.

Step 3: Make the Creamy Mushroom Sauce

1. Preheat your oven to 375°F (190°C).

2. In a large, high-sided skillet or Dutch oven, melt the 4 tablespoons of butter over medium-high heat.

3. Add the sliced mushrooms and cook, stirring occasionally, for about 8-10 minutes. Don’t overcrowd the pan (cook in batches if necessary). Allow the mushrooms to release their liquid and then brown deeply. This browning is essential for developing a rich, earthy flavor.

4. Add the finely chopped onion to the skillet with the mushrooms and cook for another 4-5 minutes, until the onion is soft and translucent.

5. Stir in the minced garlic and fresh thyme, and cook for one more minute until fragrant.

6. Sprinkle the all-purpose flour over the mushroom and onion mixture. Cook, stirring constantly, for 1-2 minutes. This cooks out the raw flour taste and creates the roux.

7. Slowly, while whisking constantly, pour in the whole milk and heavy cream. Continue to whisk to prevent lumps from forming.

8. Bring the sauce to a simmer. It will thicken as it heats. Let it simmer for 2-3 minutes until it has reached a gravy-like consistency that coats the back of a spoon.

9. Remove from the heat and stir in the soy sauce (or Worcestershire), salt, and black pepper. Taste the sauce and adjust the seasoning if necessary. It should be well-seasoned and savory.

Step 4: Assemble and Bake

1. Add the blanched and dried green beans to the skillet with the creamy mushroom sauce. Gently fold everything together until the beans are evenly coated.

2. Pour the green bean and sauce mixture into a 9×13-inch baking dish (or a similar 3-quart casserole dish) and spread it into an even layer.

3. Sprinkle the prepared crispy topping evenly over the entire surface of the casserole.

4. Bake, uncovered, for 25-30 minutes, or until the sauce is hot and bubbly around the edges and the topping is deeply golden brown and crisp.

5. Let the casserole rest for at least 10 minutes before serving. This allows the sauce to set slightly.