Preheat your oven to 375°F (190°C). Bring a large pot of salted water to a boil. Add the green beans and blanch for 5 minutes until bright green but still crisp. Immediately transfer to an ice bath to stop the cooking process. Drain well and set aside.
In a large skillet or Dutch oven, cook the diced bacon over medium heat until crispy, about 8-10 minutes. Using a slotted spoon, transfer the bacon to a paper towel-lined plate, leaving about 2 tablespoons of bacon grease in the pan.
Add the diced onions to the bacon grease and sauté until translucent, about 5 minutes. Add the minced garlic and cook for another 30 seconds until fragrant.
Add sliced mushrooms to the skillet and cook until they release their moisture and begin to brown, approximately 5-7 minutes. The mushrooms should reduce in volume by about half and develop a deep golden color.
Sprinkle flour over the mushroom mixture and stir constantly for 2 minutes to cook out the raw flour taste. Slowly pour in the chicken broth while whisking continuously to prevent lumps from forming. Add the heavy cream, sour cream, Worcestershire sauce, thyme, and nutmeg. Simmer gently until the sauce thickens enough to coat the back of a spoon, about 5-8 minutes.
Fold the blanched green beans and cooked bacon into the creamy mushroom sauce, ensuring everything is well coated. Transfer the mixture to a 9×13-inch baking dish, spreading it evenly.
In a small bowl, combine the French-fried onions, Panko breadcrumbs, grated Parmesan cheese, and melted butter. Sprinkle this mixture evenly over the green bean mixture.
Bake uncovered in the preheated oven for 25-30 minutes until the topping is golden brown and the sauce is bubbling around the edges. For the final 5 minutes, you may want to place the casserole under the broiler to get an extra crispy top—just watch carefully to prevent burning.