Moroccan Sweet Potato Soup

• 2 tbsp extra virgin olive oil
• 3 medium sweet potato, chopped into small pieces
• 2 zucchini
• 400 g tin lentils, drained
• 1 l salt reduced vegetable stock
• 2 tbsp Tbsp Moroccan spice
• 2 cup water
• 3 tbsp plain Greek yoghurt, to serve (optional)
• 2 tbsp pumpkin seeds to garnish (optional)