Preheat oven to 325 degrees F (165 degrees C). Grease a 9x13 inch baking dish.
Melt the butter in a small sauce pan over medium-low heat.
When melted, whisk in the flour and continue whisking until smooth and bubbly. Remove from the heat and slowly whisk in the chicken broth and milk.
Return to the heat and bring to a gentle boil, whisking constantly, until thickened.
Add salt and pepper Remove from and and set aside.
Mix the corn flake crumbs, parmesan cheese, and 1/4 cup melted butter in a large bowl, then spread about 2/3 of the crumb mixture into the bottom of the prepared baking dish.
Layer half the chicken over the corn flake crumbs and sprinkle with salt and black pepper.
Cover with half the Swiss cheese, then a layer of ham, followed by layers of mushrooms and green onions.
Mix the sauce you put aside to the sour cream together in a bowl, and spread the mixture over the green onion layer.
Place the rest of the chicken onto the 1st layer, followed by the rest of the Swiss cheese. Top the casserole with the remaining 1/3 of the crumb mixture.
Cover the casserole with aluminum foil, and bake in the preheated oven for 45 minutes remove the foil and bake an additional 15 minutes until the casserole is bubbling and hot in the middle and corn flakes are browned.