Parboil the potato slices. Bring a large pot of salted water to a boil and add the sliced potatoes. Cook for 5 minutes, just until they start to bend without breaking not fully tender, just partway there. Drain well. This is the step that fixes the raw-center problem most scalloped potato recipes run into: starting the potatoes partly cooked means they only need to catch up to the sauce in the oven instead of racing to cook through from completely raw, which is what usually forces you to either extend the bake time (and scorch the top) or serve slices that are still firm in the middle.
Make the sauce. Melt the butter in a saucepan over medium heat. Whisk in the flour and cook for about 1 minute, until it smells lightly toasted and no longer raw. Gradually whisk in the milk and chicken broth, a little at a time, until smooth. Simmer, whisking often, for 4–5 minutes until the sauce thickens enough to coat the back of a spoon.
Finish the sauce. Remove the saucepan from the heat and stir in 1½ cups of the cheddar, the parmesan, garlic powder, onion powder, salt, and pepper. Stir until smooth the residual heat is enough to melt the cheese without it needing to hit direct heat again, which keeps the sauce from turning grainy.
Layer the dish. Preheat the oven to 375°F. In a greased 9×13-inch baking dish, layer half the parboiled potatoes, half the ham, and half the sauce. Repeat with the remaining potatoes, ham, and sauce. Pro tip: press each layer down gently as you go it helps the sauce distribute evenly instead of pooling only at the top.
Top and bake. Sprinkle the remaining ½ cup of cheddar over the top. Bake, uncovered, for 35–40 minutes, until the top is golden and bubbling and a knife slides easily through the center.
Rest before serving. Let the dish sit for 10 minutes before serving it firms up slightly as it cools, which makes for cleaner slices instead of a loose, saucy scoop.
Serve. Top with chopped chives or parsley and serve warm.
