Place the onion in a large saucepan with 300 ml vegetable stock and gently simmer for approx. 10 min.
Meanwhile, stir ginger, turmeric, cinnamon, cayenne pepper, and 2 tbsp stock into a paste. Mix into the onion mixture with the carrots, celery, and remaining stock.
Allow the mixture to bubble up and simmer on low heat for 5 min with the lid on.
Add tomatoes and potatoes and simmer gently with lid closed for 20 min.
Fold in coriander, peas, saffron and juice of one lemon. Season with salt and pepper and serve hot.