• Preheat the oven to 375°F and grease a 9x13-inch baking dish.
• Wash and dry your potatoes. Then, slice using a mandoline. If your potatoes are very large (like a bakers potato), you can cut them in half lengthwise.
• Set potatoes aside while preparing the sauce, which takes about 10 minutes. 
• Melt butter in a large skillet over medium heat. Add onion; cook and stir until starting to soften, 3 to 4 minutes. Sprinkle flour over onion and whisk together. Cook until golden brown, about 2 minutes. 
• Stir in half-and-half and milk; whisk until thickened, 3 to 4 minutes more.
• Remove sauce from the heat. Season with salt and pepper, garlic powder, Worcestershire sauce, onion powder, and paprika.

When Ready to Assemble

• Place half the sliced potatoes into a prepared baking dish; top with half the chopped ham, ½ the Cheddar cheese and ½ the Monterey Jack cheese. 
• Pour in half the sauce from the skillet. 
• Repeat layering the remaining potatoes. Add the rest of the ham and then pour over the rest of the sauce. Cover with remaining cheeses.
• Cover with aluminum foil and bake in the preheated oven for 40 minutes.
• Remove the foil and bake until the cheese on top is golden and sauce is bubbling, 20 to 30 minutes more.
• Serve warm.