Step 1

Bring stock to the boil, then pour over couscous in a heatproof bowl. Stir, cover and set aside for 5 minutes.

Step 2

Heat 2 teaspoons oil in a frypan over medium heat. Cook onion for 2-3 minutes until soft. Add garlic and cook for 1 minute or until fragrant, then place in a food processor with the beans, harissa paste, mint, zest and prepared couscous. Pulse until mixture just comes together. Season to taste with salt and pepper, then form into 4 patties and chill for 10 minutes.

Step 3

Heat the remaining 1 1/2 tablespoons oil in a frypan over medium heat and cook patties for 2 minutes each side until crisp. Spread half the bread slices with hummus. Top with lettuce, tomato, patties, yoghurt, mint and remaining bread.