Peel and finely chop the onion. Peel and grate or crush the garlic. Finely chop the parsley stalks (keep the leaves for later).
Warm a large pan for 1 min. Add ½ tbsp olive oil and the onion. Season with salt and pepper. Cook over a medium heat for 5 mins, stirring now and then, till the onion is soft.
While the onion cooks, halve the pepper. Scoop out the seeds and white bits. Roughly chop it. Roughly chop the tomatoes.
Stir the garlic and parsley stalks into the onion. Cook and stir for 1 min. Add the green pepper and tomatoes. Cook for 3-4 mins, stirring now and then, till the veg have softened a little.
While the veg soften, scrub the potatoes and chop them into bite-size pieces. Dissolve the stock cube in 1 ltr boiling water.
Add the potatoes to the pan of veg. Pour in the hot stock. Cover, bring to the boil, then turn the heat down. Simmer for 15 mins till the potatoes just start to soften.
Add the fish pie mix to the pan. Cover and simmer for 4-5 mins to cook the fish – it should be opaque and flake easily. Taste the stew and add more salt and pepper if it needs it.
Ladle the stew into a couple of warm bowls. Roughly chop the parsley leaves and scatter them over the stew to serve.