Creating Creamy Classic Scalloped Potatoes is simple when you follow these steps:

1. Preheat the Oven: Preheat your oven to 375°F (190°C).
2. Prepare the Baking Dish: Grease a 9×13-inch baking dish with butter or cooking spray.
3. Sauté the Onion and Garlic: In a skillet, melt 2 tablespoons of butter over medium heat. Add the chopped onion and minced garlic, sautéing until softened, about 3-4 minutes.
4. Make the Cream Sauce: In a separate saucepan, melt the remaining 2 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 minute. Gradually add the heavy cream and milk, whisking continuously until the mixture thickens. Stir in the salt, pepper, and paprika.
5. Layer the Potatoes: Begin layering half of the sliced potatoes in the prepared baking dish. Pour half of the cream sauce over the potatoes, followed by half of the cheddar and Parmesan cheese.
6. Repeat the Layers: Add the remaining potato slices on top, followed by the rest of the cream sauce. Finally, top with the remaining cheeses.
7. Cover and Bake: Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes.
8. Uncover and Continue Baking: Remove the foil and bake for an additional 15 minutes, or until the potatoes are tender and the top is golden brown and bubbly.
9. Let it Cool: Remove from the oven and allow to cool for 10 minutes before serving. This will help the dish set up a bit.
10. Garnish: If desired, sprinkle with fresh parsley before serving for a pop of color.

These steps will guide you effortlessly in creating this incredible dish of Creamy Classic Scalloped Potatoes.