Preheat your oven to 350°F (175°C).

In a large skillet over medium-high heat, add the lean ground beef and chopped onion. Cook for about 5-7 minutes until browned and onions are translucent, then season with salt and pepper and transfer to a bowl.

Melt 4 tablespoons of butter in the same skillet over medium heat. Whisk in ¼ cup of flour and cook for about 2 minutes. Gradually add 2 cups of milk, stirring continuously until the sauce thickens. Mix in paprika, mustard powder, dried parsley, and salt & pepper, then fold in 1.5 cups of cheddar cheese until melted.

In a 9x11-inch casserole dish, layer half of the sliced russet potatoes at the bottom, then half of the cooked beef mixture, followed by one-third of the cheesy sauce.

Repeat the layers with the remaining potatoes, beef, and sauce, finishing with the last layer of potatoes and pouring the remaining sauce on top.

Cover the casserole tightly with aluminum foil and bake for 1.5 hours, removing the foil 20 minutes before the end to let the cheese top brown.

Let the casserole cool for about 10-15 minutes before serving to allow it to set.