Preheat the oven to 400°F. Lightly grease a 12-inch cast iron skillet or 9-x-13-inch casserole dish.
In a large saucepan, warm the milk and butter over medium heat until the butter has melted. Add the flour and cook just until it thickens to a thin gravy.
Remove from the heat and stir in the onion dip until smooth.
Place half of the potatoes on the bottom of the skillet or dish. Pour half of the milk mixture on top. Sprinkle with 1 cup of ham, 1 cup mozzarella cheese and ½ cup Parmesan cheese. Salt and pepper, to taste. Repeat the layers.
Cover with foil and bake for 45 minutes. Remove the tinfoil and continue cooking an additional 20 minutes, or until the potatoes soften and the cheese has slightly browned. You can brown the cheese more by broiling for a couple minutes or moving it to the top rack at the end of baking. Serve warm.
Tip: We prefer the Hiland brand onion dips.