1. Heat the oil in a flameproof casserole dish or large, lidded saucepan over a medium heat and fry the onion, carrot, celery and garlic for 12-15 mins until softened. Meanwhile, halve the tomatoes widthways and coarsely grate using a box grater set over a bowl (discard the skin). Add to the pan with the parsley stalks, potatoes and 250ml water.
2. Season well, partially cover the pan and bring to a brisk simmer. Cook for 20 mins, stirring occasionally, or until thickened slightly and the potatoes are tender.
3. Add the frozen fish, cover and return to a simmer. Cook for 10-15 mins or until the fish flakes. Stir in the lemon zest and the juice of 2 lemon wedges, gently breaking up the fish a little with the spoon. Scatter with the celery and parsley leaves. Drizzle with a little more oil and serve with crusty bread, if you like, and the remaining lemon wedges for squeezing over.
Tip: You can make this in a slowcooker: add all the ingredients apart from the celery and parsley leaves, fish and lemon; cook on high for 45 mins- 1 hr. Add the fish and lemon and cook for another 15-20 mins.
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