Fry the onion in a large frying pan in a splash of olive oil until golden brown. Add the garlic and fry for another 30 seconds then remove from the heat. Allow to cool to room temperature.
Mix the cooled onions with the beef, parsley, breadcrumbs, egg, salt and pepper. Adjust the seasoning to taste.
Divide the mixture into 5-6 (depending on how you big you want the patties), and form patties with wet hands.
Place on a tray lined with baking paper and place in the fridge to chill for 30 minutes.
When the patties have rested, heat a large cast iron pan/griddle pan until hot then cook the burgers until charred and cooked to your preference.
Remove from the heat, top with the cheese and cover with foil for 2 minutes.
Toast the buns then add a spoonful of mayonnaise or any sauce/condiment of your choice. Add lettuce, the burger patties, cheese, crispy bacon and sliced tomatoes then serve.
Notes
To test the seasoning, cook a small piece of the mixture in a hot pan, taste and adjust as necessary.
Wet your hands before forming the patties to prevent the mixture from sticking.
These burger patties can be cooked on a grill (charcoal grill/ gas grill will both work well) or in a cast iron skillet set over medium-high heat.