Preheat your Oven: Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Prepare the Meatball Mixture: In a medium-sized mixing bowl, combine 1 pound of ground beef , ½ cup of finely chopped sweet yellow onion, ⅓ cup of plain panko bread crumbs, 1 beaten extra-large egg, 2 teaspoons of Italian seasoning, 1 teaspoon of kosher salt, 2 teaspoons of fresh chopped parsley (reserve 1 teaspoon for garnish), and ½ teaspoon of fresh cracked black pepper. You can mix using a spoon or your hands, making sure everything is well combined.
Form the Meatballs: Use a 1½ tablespoon scoop to portion out the mixture and roll it into balls. Place the meatballs on the prepared baking sheet, spacing them about 2 inches apart. You should yield around 22-23 meatballs and bake them in the preheated oven for 15 minutes.
Prepare the Potatoes: While the meatballs are baking, bring a pot of water to a boil. Add 5 medium-sized Yukon Gold potatoes and cook for 15-20 minutes until fork-tender. Drain and rinse them under cold water to stop the cooking process.
Make the Tomato Sauce: In a small mixing bowl, combine 1½ cups crushed tomatoes, 1 teaspoon Italian seasoning, ½ teaspoon kosher salt, and ½ teaspoon cracked black pepper. Stir well and set aside.
Prepare the Cream Sauce: In a heavy-bottomed saucepan over medium-high heat, melt 4 tablespoons of salted butter. Once melted, whisk in 5 tablespoons all-purpose flour continuously for about 1-1½ minutes. Gradually whisk in 2¼ cups of warmed whole milk, stirring until the sauce thickens. Remove from the heat and mix in ½ teaspoon kosher salt, ½ teaspoon fresh cracked black pepper, and ⅛ teaspoon ground nutmeg.
Layer the Dish: Remove the baked meatballs from the oven. Slice the potatoes into ⅛ inch thick slices. In a greased 9 x 9 baking dish or a cast-iron skillet, layer the potato slices to cover the bottom and sides. Next, place the meatballs on top and create a design that forms a “bullseye.” Fit the remaining potato slices around the meatballs.
Assemble the Sauces: Spoon the prepared tomato sauce evenly over the meatballs. Then, pour the béchamel sauce over the tomato sauce, ensuring you use all the sauce. Finish by sprinkling 1 cup of freshly shredded mozzarella on top.
Bake the Dish: Bake the casserole for 45 minutes, or until the cheese is golden brown.
Rest and Serve: Allow the dish to rest for approximately 10-15 minutes before serving. This lets the flavors meld and allows the dish to cool slightly. Garnish with the reserved 2 teaspoons of chopped parsley and serve warm.