Spiced lentil and kumara pies

Ingredients

• 2 tablespoons olive oil
• 1 medium red onion (170g), chopped finely
• 3 cloves garlic, chopped finely
• 1 celery stalk (150g), trimmed, chopped finely
• 2 tablespoons harissa (see tips)
• 1 teaspoon ground cumin
• 1 teaspoon ground coriander
• 2¼ cups (450g) french-style green lentils
• 2 small kumara (orange sweet potato) (500g), cut into 3cm pieces
• 2 cups (500ml) vegetable stock
• ½ cup (125ml) water
• 400 grams canned cherry tomatoes in juice
• 60 grams baby spinach leaves
• ½ cup fresh flat-leaf parsley leaves
• ½ cup fresh coriander leaves
• 1½ teaspoons finely grated lemon rind
• 4 sheets puff pastry
• 1 free-range egg, beaten lightly
• Herb & lemon yoghurt
• 1 cup (280g) Greek-style yoghurt
• ¼ cup coarsely chopped fresh flat-leaf parsley
• ¼ cup coarsely chopped fresh coriander (cilantro)
• 1 tablespoon finely chopped preserved lemon rind
• 1 tablespoon lemon juice