Bring a large pot of water to a boil. Blanch green beans for 3-4 minutes, then remove with a slotted spoon to an ice bath. Let sit for a few minutes, then cut green beans into 2 inch pieces and place in a large bowl.
Melt 2 tablespoons of butter in a large skillet. Add mushrooms and onions and cook for 5 minutes, until onions are softened. Then stir in garlic, salt and pepper and cook for another minute.
Transfer mushroom mixture to the bowl with the green beans and set aside.
Melt 2 more tablespoons of butter in the skillet, then whisk in the flour until no lumps remain. Slowly pour in the milk while whisking until combined and not lumpy. Then whisk in the sour cream and stir until thickened, 2-3 minutes. Pour the sauce on top of the green beans and mushrooms and toss to combine.
Transfer the mixture to an 8x8 baking dish sprayed with non-stick spray. Preheat the oven to 350 degrees.
Melt the final tablespoon of butter in the skillet and add in the Panko and parmesan cheese. Stir to combine and cook until lightly toasted, about a minute. Spread the mixture on top of the green bean mixture.
Bake for 25 minutes or until topping is browned and crispy.
To save on dishes, you can wipe out the skillet between each step. If you'd like a cheesier casserole, top it with shredded gruyere or mozzarella and the bread crumb mixture. For step by step tips on blanching green beans, view my How to Blanch Green Beans recipe.