Mix the ingredients for the sweet potato bun (apart from the oil) in a large mixing bowl and divide into 8 patties. Heat the oil in a non-stick frying pan, and fry the patties for 3 minutes each side or until browned.
Mix the beef burger ingredients (apart from the oil) in a mixing bowl, divide into 4 and shape into 4 large thin burgers. Add the oil to the non-stick frying pan and cook the beef burgers for 4-5 minutes each side, until cooked through.
Layer the spinach, avocado, beef burger and caramelised onion on a sweet potato patty, and top with a second sweet potato patty.
Meanwhile, mix the kale with all of the ingredients in a bowl, spread evenly on a baking tray and bake in a preheated oven 200°C/ 180°C fan / Gas 5 for around 3 minutes until crisp. Serve with the beef burger.