Heat the oil in a large saucepan and fry the onion for 3-4 minutes, add the garlic, coriander and turmeric and fry for 1-2 minutes. Dissolve the stock cubes in 1.5 litres boiling water. Stir the split peas, 50g Walnuts and stock into the pan, cover and simmer for 45 minutes or until the split peas are tender, season to taste. Puree with a stick blender until smooth.
Stir together the yogurt, harissa, half the chilli and half the remaining Walnuts.
Pour the soup into bowls and drizzle with the harissa yogurt, sprinkle with remaining chilli and Walnuts to serve.
Cooks tip
For a coarse texture to the soup, blend half and stir back into the rest of the soup. Try adding chopped coriander just before serving.