1. To make the avocado, halve the avocado and remove the stone. Scoop out the flesh and place in a blender with the lemon juice, chilli, sugar and salt. Blend until smooth.
2. Make the carrot salad by peeling the carrots, and then cutting into long, thin strips. Combine the yoghurt, garlic, salt and pepper. Stir it into the carrots.
3. Prepare the beef by dividing the meat into 4 portions and forming into large flat patties. Brown them in butter and oil in a very hot frying pan for about 1 min. on each side. Season with salt and pepper. Continue cooking for an additional 2-3 min. on each side over a medium heat. The meat should be cooked to medium and the juices should run clear.
4. Assemble your sandwich: Toast the bread slices using a toaster or hot grill pan. Spread avocado cream on the bread slices. Layer on lettuce, hot beef patty, tomato slices and onion slices. Top with a second slice of toasted bread – and press down slightly. Serve with the carrot salad and any remaining avocado cream.