Finely chop the fennel and onion. Put the fennel fronds aside.
Heat the oil in a pan, add the chopped vegetables and gently cook until tender.
Meanwhile peel the potatoes, cut into small cubes, then add to the. Stir for a minute, then pour in the passata and water. Add the bay leaf and a couple of pinches of salt.
Bring to the boil, then put the lid on and simmer for 20 minutes on a low heat.
Turn off the heat then blend the soup until creamy.
Add the prawns, warm it up and serve with the chopped herbs and fennel fronds. Season to taste.