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.
Recipe courtesy of Nadia G