Preheat the oven to 230c/450f and prepare a large baking tray.
Peel the onion and slice into thick slices. Separate the slices into individual rings.
Place the flour, garlic salt, and black pepper into a bowl and mix.
Break the egg into another bowl and whisk lightly with a fork.
Place the parmesan into a third bowl and add the breadcrumbs and fajita seasoning. Mix together and then drizzle over the olive oil. Mix in the oil so the breadcrumbs are all covered in a little oil.
Dip the onion rings into the flour mixture, followed by the egg and finally the breadcrumbs before placing onto the baking tray. You may need to sprinkle on a few more breadcrumbs once on the tray to make sure they're fully covered in the breadcrumbs. Repeat with the other onion rings. I tend to only use the larger onion rings (saving the smaller rings for another recipe).
Place the onion rings in the oven and cook for 12-15 minutes until golden brown and crunchy, then take out of the oven.
Meanwhile, heat a griddle or large frying pan until hot and brush with the vegetable oil.
Mix together the minced beef, pepper and salt in a bowl and form the mince into 4 patties. Place the patties on the griddle and cook, turning once until well browned on the outside and no-longer pink in the middle (about 5-6 minutes). If you want the cheese melted, add the cheese on top as soon as you turn the burgers over. Take the burgers off the griddle when cooked and leave to rest for 2-3 minutes.
Whilst the burgers are resting, place the bacon rashers on the griddle and cook for 2-3 minutes, turning once, until crisp.
Mix the sauce ingredients together and put to one side.
Now it's time to assemble the burgers. Place the lettuce on the bottom of each of the burger buns. Add the burger to each bun, followed by slices of tomato. Top each with 2 or 3 onion rings and two rashers of bacon. Place the top of the bun on the burger and serve.