Add the thinly sliced red onion to a sauté pan over medium heat until soft. Season with salt and pepper. Set aside.
In the same pan, cook the bacon to your desired doneness and set aside.
In a bowl, mix together the ground sirloin, the Heinz 57 sauce, and Montreal steak seasoning until well combined.
Form the hamburger mixture into 4 regular-sized patties and keep in the fridge until you are ready to cook them. You could make the patties slider-sized if you wish.
Set up your burger topping station: bacon strips, sautéed onions, cheese slices, lettuce, tomato slices, and buns.
Preheat the grill to medium-high. Remove burgers from fridge. Once grill is hot, cook burgers to your desired doneness. Optionally, you can also toast the buns on the grill.
Assemble your burger with toppings and enjoy.