In a large pot, heat oil and add the onions, celery and carrots. Cook on high heat for 5 minutes with regular stirring.
Then add the bay leaf, cumin powder, coriander powder, chilli flakes, saffron with milk and vegetable stock. Season with salt and mix well to combine.
Bring to boil and then simmer gently till the carrots are completely cooked (takes about 20-25 minutes).
Cool slightly and add the lemon zest. Blend into a soup like consistency using a stick blender. If using a mixer, blend in batches.
Return to heat and add half of the lemon juice. Taste and add more if necessary. Also add water to get the desired consistency if the soup feels too thick.
Serve warm garnished with croutons, fresh coriander leaves and a pinch of sweet paprika.