Peel and finely chop the onion. Trim and peel the carrot. Chop it into small dice. Trim the dry ends off the celery and dice it to match the carrot and onion.
Warm a medium pan for 1 min, then add ½ tbsp olive oil and the diced veg. Season with salt and pepper. Pop on the lid. Keep the heat low and gently cook the veg for 8-10 mins, stirring now and then, till they’re glossy.
While the veg sweat, peel and chop the potatoes into chunks around 2 cm across. Peel and crush or grate the garlic. Finely chop the parsley stalks, chop the leaves and put to one side.
Crumble the stock cube into a heatproof jug and stir in 450ml boiling water till the stock cube dissolves.
Add the garlic and parsley stalks to the veg. Cook and stir for 1 min. Add the potatoes. Tip in the tin of tomatoes. Pour in the stock. Cover, turn up the heat and bring to the boil.
When the stew starts boiling, turn the heat down to medium-low and simmer for 12-15 mins till the potatoes are just cooked when pressed with a plum fork. Give the stew a good stir to break up the tomatoes.
Add the fish pie mix to the pan. Cover and simmer for 3-5 mins to cook the fish through - it should be opaque and either pink (for salmon) or white (for the white fish).
Taste the stew and add more salt and pepper if you think it needs it. Ladle into warm bowls and serve with parsley leaves. scattered on top.