Peel and finely slice the onion. Trim the fennel and cut into 1 cm slices. Reserve any green fronds for later. Peel and crush the garlic. Halve the chilli. Flick out most of the seeds. Finely chop.
Cut the stem from the squash. Cut it in half lengthways (no need to peel). Scoop out the seeds with a spoon and discard. Coarsely grate the squash. If you have a food processor with a grating attachment, use that to make this extra quick.
Pour 50 ml water into a large pan. Add the onion and sliced fennel. Fry for 3-4 mins till just softened. Add 2 tsp fennel seeds, the crushed garlic and chilli to the pan. Cook for 1 min.
Pour in the passata. Fill the jar twice with cold water (around 800 ml). Add to the pan. Bring to a simmer, then add the grated squash. Pop on a lid and cook for 15 mins.
Meanwhile, cut any large pieces of fish in half. Roughly chop the parsley leaves. Zest and juice the lemon.
When the stoup has simmered for 15 mins, add in the fish pie mix. Pop the lid back on and continue to cook over a low heat for 5 mins till the fish is opaque and cooked through. It should flake easily when pressed with a fork.
Pour in half of the lemon juice. Taste and adjust the seasoning. Add more lemon juice to taste. Ladle the fish stoup into warm bowls. Scatter over the lemon zest, the chopped parsley and any fennel fronds to serve.