Heat your oven to 200°C/Fan 180°C/Gas 6. Peel and finely chop the red onion. Halve the pepper. Scoop out the seeds and white bits. Finely chop it.
Warm a deep frying pan over a medium heat for 1 min. Add ½ tbsp olive oil, the onion and pepper. Season with a pinch of salt and pepper. Cook for 5 mins, stirring occasionally, till they start to soften.
While the veg cook, peel and crush the garlic. Finely chop the stalks from the parsley sprigs (keep the leaves for later). Roughly chop the olives.
Stir the garlic, parsley stalks and olives into the frying pan. Pour in the tomatoes. Half fill the tomato tin with water (around 200ml) and add to the pan. Cover, bring to the boil, then turn the heat down and simmer for 20 mins to mingle the flavours and start the sauce thickening.
While the sauce simmers, scrub the potatoes and slice them into thin chips around ½cm thick. Pop them in a bowl and toss them with ½ tbsp olive oil and a pinch of salt and pepper. Line a baking tray with baking paper. Spread out the chips on the
tray. Bake for 20 mins till golden and tender
After the sauce has simmered for 20 mins, add the haddock fillets to the pan. Cover and cook for 5-8 mins till the haddock is white, opaque and flakes easily when you press it with a fork.
Roughly chop the parsley leaves. Serve the haddock with the tomato sauce and the frites.