Preheat the oven to 375°F and grease a 9x13 baking dish.
Wash and dry your potatoes and 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 and season with salt and pepper, garlic powder, Worcestershire sauce, onion powder, and paprika.