Pour stock into small saucepan and bring to the boil. Place couscous in a heatproof bowl, pour over stock and stir. Cover with cling wrap and set aside for 5 minutes. Remove cling wrap and fluff couscous with a fork.
Heat 1 tablespoon of olive oil in small frying pan over medium heat. Cook the onion for 2–3 minutes until soft. Add the garlic and crushed chilli, cook until fragrant, about 2 minutes.
Place the beans in a food processor, add the onion mix, mint and lemon zest. Pulse until mixture combines together. Place mixture in couscous bowl and fold through. Season with salt and pepper to taste, form into four patties, cover and chill for 30 minutes.
Heat the remaining 2 tablespoons oil in frying pan over medium heat and cook patties for 2 minutes each side. Remove from pan and drain on paper towel.
Meanwhile, toast bread and spread one side of each slice with hummus, top four of the slices with some arugula, pineapple, beetroot, a patty and a dollop of yoghurt each. Top with the remaining bread.