Step 1 Mash the beans.

Step 2 Heat olive oil in pan, add the grated kumara and cook for 4-5 minutes or until soft. Add this mixture to the beans in a bowl, then add the red onion, chilli flakes and garlic.

Step 3 Lightly beat the egg yolks and combine with the bean and kumara mixture.

Step 4 In another bowl, whisk the egg whites together, fold into the kumura mixture, then form into 4 patties and place on a baking tray.

Step 5 Preheat the grill and coat the upper side of the patties with cooking oil spray. Cook patties under the grill for 3-4 minutes or until golden, then turn over, coat with cooking oil spray again and grill the other side for a further 3 minutes, or until golden.

Step 6 Place lettuce leaves on the slices of toasted bread, top with burgers and finish with mayonnaise, tomato, cheese and cucumber slices.