Portion the ground brisket into eight 3½-ounce (100 g) balls. Place the balls between two squares of parchment and squish them super flat with a flat plate or the bottom of a pan.
In a medium stockpot, melt the butter in the olive oil over medium heat. Add the onions and cook low and slow until they start to caramelize and get creamy, about 20 minutes. Deglaze with the maple syrup. Season with salt and add the vinegar. Reserve warm until it’s time to build the melts.
Heat the vegetable oil in a large cast-iron skillet over high heat. Once the oil is hot, throw in the smashed beef patties, season with salt and pepper, then flatten them out with a spatula. Cook on one side 95 percent of the way through, flip with spatula, then spoon on the cheese and flatten the cheese out. Add a splash of water to the pan, cover, and let the cheese melt.
Butter both sides of the molasses bread and toast the slices in a dry skillet over medium-high heat. Add more butter to the pan when you flip the bread. Transfer to a plate, place a patty on the bottom slice of bread, then add another patty on top of that. Spoon a big scoop of caramelized onions on the top patty, add mustard, and throw on the top slice of bread. Repeat until you have the desired number of patty melts for your friends.