Scrub the potatoes and halve or quarter any large potatoes so they’re all roughly the same size. Slice a small cross into the bottom of each tomato. Pop them in a heatproof bowl. Cover with boiling water. Put to one side for 10 mins.
Peel and finely chop the onion. Peel and grate or crush the garlic. Warm ½ tbsp olive oil in a pan. Add the onion. Season. Cook over a lowish heat for 8-10 mins till the onion is soft but not too coloured. Stir often.
Drain the tomatoes. You should be able to pull off the skins. Halve them and scoop out the seeds. Roughly chop the tomato flesh.
Stir the garlic and tomatoes into the onion with a pinch of harissa and 1 tsp of the cumin. Cook and stir for 2 mins till the pan smells aromatic.
Add the potatoes to the pan. Pour in 800ml boiling water. Cover. Bring to the boil, then turn the heat down and simmer for 15 mins so the potatoes are almost cooked. They should still be a little firm if pressed with a fork.
Add the fish pie mix to the pan. Put the lid back on. Simmer for 5 mins to cook the fish pie mix and finish cooking the potatoes.
Finely grate or pare the zest from the lemon. Juice it. Stir the juice into the stew. Taste and adjust the seasoning. Shred the larger mint leaves.
Serve the stew in warm bowls topped with the lemon zest and mint leaves, sprinkled with a pinch more harissa if you like extra spice.