Divide the meat mixture into balls of 1/3 lb each (150 grams or so). Place each on a 6" square section of wax paper or parchment
Heat a cast iron skillet or stainless steel griddle to medium-high heat. Invert each ball of meat onto the hot surface and, leaving the paper in place, use a flat weight or a large metal spatula to press the ball into a circular patty roughly the size of your hamburger buns.
Remove the paper and season well with salt and pepper
Cook on the first side until nicely browned then add a small pile of the thinly-sliced onions and turn the patties over.
Douse with the sauce. Allow to cook to medium, turning and basting with more sauce as needed.
Place cooked patty with onions onto bottom half of bun, scooping up as much of the hot sauce from the griddle as you can get. Add additional raw diced onions if desired. Top with a large handful of shoestring potato fries. Drizzle more sauce on top of the shoestring fries. Add ketchup to top bun if desired.