First make the harissa yogurt by stirring the harissa paste, a squeeze of lime juice and the thick yogurt together. Set aside.

Place the butter in a heavy-based saucepan over a medium heat. Sauté the spring onions for 2-3 minutes, until lightly coloured. Add the hot stock and increase the heat. Add the mixed lettuce, baby spinach and herbs and stir to wilt. After 2 minutes, blend the soup until smooth. Add the cream and salt and pepper to taste. Serve immediately with a dollop of harissa yogurt before the lovely green colour fades.