Start by washing and peeling your russet potatoes. Using a sharp knife or mandoline slicer, thinly slice the potatoes about 1/8 inch thick. Finely chop the onion and mince the garlic cloves.
In a large skillet, melt the butter over medium heat. Add the chopped onion and sauté until softened and translucent, about 5 minutes. Stir in the minced garlic and cook for another 1-2 minutes, until fragrant but not browned.
Pour the heavy cream and chicken broth into the skillet with the onions and garlic. Stir in salt, black pepper, paprika, and nutmeg. Bring the mixture to a gentle simmer, stirring occasionally. Remove from heat once it’s warmed through and the flavors have melded.
Preheat your oven to 350°F (175°C). Lightly grease your 9x13 inch baking dish. Arrange a layer of sliced potatoes on the bottom, slightly overlapping. Sprinkle a portion of the shredded cheddar and mozzarella over the potatoes. Repeat layering with the remaining potatoes and cheeses, finishing with a cheese layer on top.
Slowly pour the creamy sauce evenly over the layered potatoes and cheese, allowing it to seep through the layers. Cover the dish tightly with aluminum foil and bake in the preheated oven for 60 minutes.
After 60 minutes, remove the foil and continue baking uncovered for an additional 20-25 minutes. This will let the top cheese layer bubble and brown to a perfect golden finish.
Once out of the oven, let the casserole rest for 10 minutes to set. Sprinkle the chopped fresh parsley over the top for a burst of color and fresh flavor. Serve warm and enjoy!