Step 1: Preheat the Oven and Prepare the Dish

Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter to prevent sticking.

Step 2: Prepare the Potatoes

Peel the potatoes and slice them thinly (about ⅛ inch thick). If you have a mandoline slicer, this will make the process faster and ensure even slices.

Step 3: Make the Cheese Sauce

1. In a medium saucepan, melt 3 tbsp butter over medium heat.
2. Add the minced garlic and sauté for 30 seconds until fragrant.
3. Stir in the flour, whisking constantly for 1-2 minutes to cook out the raw taste.
4. Gradually whisk in the milk and chicken broth, stirring continuously until smooth.
5. Add the salt, black pepper, paprika, and onion powder, mixing well.
6. Bring the sauce to a gentle simmer, then reduce the heat and stir in cheddar and Parmesan cheese until fully melted. Remove from heat.

Step 4: Layer the Potatoes

1. Arrange half of the sliced potatoes in an even layer in the prepared baking dish.
2. Pour half of the cheese sauce over the potatoes, spreading it evenly.
3. Repeat with the remaining potatoes and cheese sauce.

Step 5: Add the Cheesy Topping

1. Sprinkle mozzarella and Parmesan cheese over the top.
2. Cover with foil and bake for 40 minutes.

Step 6: Bake Until Golden

1. Remove the foil and bake for another 15-20 minutes, or until the top is golden brown and bubbly.
2. Let the potatoes rest for 10 minutes before serving.

Step 7: Garnish and Serve

Sprinkle with fresh parsley for a pop of color. Serve warm and enjoy!