Cheesy "Funeral" Potatoes

Preheat oven to 350 degrees F. Melt 3 tbsp butter in a large skillet over medium heat. Sauté onion until translucent. Sprinkle with flour and cook for 1 minute. Whisk in milk and chicken stock. Bring to boil and let simmer to thicken. Season with salt and pepper. Remove from heat.Stir in cheeses and sour cream. In a large bowl, combine hash browns with sauce. Transfer to a buttered casserole dish. In a medium bowl, combine cornflakes and butter. Top casserole. Cover the dish and bake for 45 minutes, or until bubbly.