Slowly add in the gluten-free flour and whisk until you have a roux. Gradually add broth, and milk. Whisk until all combined and thickened. Stir in garlic powder, salt and black pepper to taste.
Follow with 1/2 cup of cheddar cheese and stir until cheese is incorporated and melted. Remove from heat. In a medium bowl combine the potatoes, ham, and pour in the sauce. Transfer to casserole dish.
Cover with aluminum foil and bake for 60-65 minutes. Meanwhile mix cornflakes with butter.
Remove foil, sprinkle with remaining 1/2 cup of grated cheese, and top with the buttered Corn Flakes. Return to the oven uncovered and bake for another 10-15 more minutes.
Let the casserole rest for about 10 minutes before serving (the sauce will thicken as it cools). Garnish with fresh parsley just before serving.