Preheat the oven to 350 F. Heat a large skillet over medium-high heat and cook the hamburger with the chopped onion until no pink is left and most of the liquid has evaporated. Season with salt and pepper and transfer to a bowl.
In the same skillet, melt the butter and add the flour. Cook the flour for about two minutes.
Whisk in the milk, cook until thick and add the paprika, dried parsley, mustard powder, salt, and pepper to taste.
Remove from heat and stir in 1 ½ cups shredded cheddar.
Place a layer of sliced potatoes in the bottom of a 9x11-inch casserole dish. Spoon half of the cooked ground beef evenly over the potatoes. Pour ⅓ of the cheese sauce on top of the meat. Repeat layers.
End with one last layer of sliced potatoes and the remaining ⅓ of the sauce. Sprinkle ½ cup shredded cheddar over the sauce, cover with foil, and bake for 1 ½ hours. Remove the foil for the last 20 mins of cooking.