Boil a pot of water. Add the spinach and cook for 2-3 minutes. Drain and squeeze to remove excess water and roughly chop.
Whizz the salad onions and garlic in a food processor until finely chopped. Add the beans (drained) and whiz again to form a coarse puree.
Add the breadcrumbs, spinach, egg yolk and a pinch of cayenne. Season and pulse together to form a paste.
Shape the mixture into 4 cakes. Grill for 5 minutes on each side until lightly browned and heated through.
Serve in rolls with tzatziki and salad leaves.
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