Pour 400ml boiling water into a measuring jug. Stir in a pinch of saffron powder to make a saffron stock. Put to one side.
Peel and finely chop the onion. Trim and finely slice the celery. Trim any tough ends off the fennel and save any fluffy fronds for garnishing. Roughly chop the fennel.
Put the onion, celery and fennel in a medium sized pan with 100ml of the saffron stock. Season and gently cook over a low heat for 5-8 mins till the veg are soft and glossy looking. Stir every so often.
While the veg cook, peel and grate or crush the garlic. Finely grate the zest from the lemon. Juice it. Finely chop the stalks from the parsley. Put the leaves to one side.
Stir the garlic, lemon zest and parsley stalks into the veg. Cook and stir for 1-2 mins till the pan smells sweet and aromatic.
Stir the saffron stock into the pan. Add the jar of passata. Cover. Turn the heat up and bring to the boil. Once it’s boiling, turn the heat down and simmer for 5 mins to just finish cooking the veg.
Add the fish pie mix to the pan. Simmer for 5 mins till the fish pie mix is opaque and cooked through. Add half the lemon juice. Taste the stew. Add more lemon juice, salt or pepper if you think it needs it.
Ladle the soup into warm bowls. Garnish with the parsley leaves and any fennel fronds you saved in step two.