Preheat the oven to 450 degrees F. Grease the bottom of an 8- by 12-inch baking pan with the butter.
For the meatloaf: Mix together the chuck, breadcrumbs, tomato sauce, sea salt, some pepper, egg, onions and potatoes in a large bowl. Form the beef mixture into a bread-like loaf and place in the center of the baking pan.
For the awesome sauce: Whisk together the tomato sauce, mustard, sugar and vinegar in a medium bowl.
Bake the meatloaf until browned, 15 minutes. Remove from the oven and pour two-thirds of the sauce on top. Reduce the oven temperature to 350 degrees F. Bake for another 1 hour 15 minutes, basting the loaf with the remaining sauce every 10 minutes. Once the meatloaf is cooked, remove it from the pan, slice it up, and serve.