If desired, whisk together the ketchup, brown sugar, and mustard for extra sauce. Set aside.
Spread the butter on the outsides of each piece of bread. Heat a grill pan over medium heat. Add two slices of bread, butter side down, to the skillet. Top with one slice of provolone cheese. Add the meatloaf slices to the other side of the pan.
Cook the meatloaf 2-3 minutes, then flip. Continue to cook until warmed through, another 1-2 minutes. At this point, the bread should be getting nice and toasty. If the bread is getting too dark too quickly, remove it from the skillet until the meatloaf is done.
Place the warmed meatloaf on top of the toasted bread and cheese. Brush with extra sauce, if desired. Top the meatloaf with pickle slices, a generous amount of French fried onions, and another piece of cheese. Sandwich with the second slice of bread, buttered side facing out.
Flip the sandwich over and cook for another 3-4 minutes, or until the second side is golden brown and toasted. Remove from the skillet and eat immediately.