Saute onion in plenty of oil on medium-low heat for 10 minutes. Add celery, leeks, carrots and sauté another 10 minutes. Add mustard, chilli and bay leaves, cook another 5 minutes. Add celeriac and stock, simmer 15-20 minutes until everything is tender. Remove bay leaves, add coconut, and puree with stick blender. Add more stock if needed, then add lemon juice and season to taste.