Combine onion mustard mayo ingredients.
Mix red cabbage, apple cider vinegar, salt and pepper together.
In a bowl combine beef mince with salt and garlic powder. Form into a large patty. Be careful not to squash the beef too tightly together otherwise it will become tough and dry during cooking.
Fry beef patty over medium heat until crispy brown, then flip burger patty and add butter to the pan. Baste patty with the butter continuously for 1 minute. Sprinkle with salt then layer cheese slice on top. Cover frying pan with a lid or aluminum foil, turn to a lower heat and let it melt the cheese for 1 – 2 minutes.
In the meantime toast brioche bun lightly. Brush insides with butter.
To build the burger, first layer red cabbage, then pickles, followed by beef cheese patty, sliced beetroot and lots of onion-mustard-mayo.