Combine the beef, onion, breadcrumbs and egg in a large bowl. Divide mince into 4 even portions and shape each portion into a 12cm disc. Place on a plate and cover with plastic wrap. Place in the fridge for 30 minutes to rest.
Heat a large frying pan over medium-high heat. Add the bacon and cook for 2 minutes each side or until golden brown and crisp. Transfer to a plate lined with paper towel.
Add the beef patties to the pan and cook for 2-3 minutes each side or until golden brown and cooked through. Place a cheese slice on each patty. Transfer to a plate.
Meanwhile, preheat the grill on high. Toast the bread rolls under grill until golden brown and lightly toasted.
Place the base of each bread roll on serving plates. Top with lettuce, tomato and bacon. Top with a beef pattie and Fehlbergs Pickled Jalapenos. Serve immediately with tomato relish and mayonnaise, if desired.