Heat your oven to 200°C/Fan 180°C/Gas 6. Add 2 tbsp olive oil to a roasting tin. Put it in the oven to warm. Fill a pan with water and bring it to the boil.
Scrub the potatoes. Halve or quarter them so they’re all the same size. Add them to the pan. Simmer for 5 mins. Drain well.
Tip the potatoes into the roasting tin. Season with salt and pepper. Roast for 15 mins.
Peel and finely slice the onion into rings. Halve the peppers. Scoop out the seeds and white bits. Slice into thick wedges. Peel and finely chop the garlic.
Add the veg and garlic to the roasting tin. Toss them to coat in the oil and mix them in with the potatoes. Roast in the oven for 10 more mins till the potatoes are nearly cooked and the veg are just starting to brown.
Turn the veg over a few times in the tin. Rest the haddock fillets on top. Return to the oven. Roast for 10-15 mins till the haddock is cooked through.
Roughly chop the sorrel. Slice the lemon into wedges. When the fish and veg are cooked through, scatter over the sorrel and serve with the lemon wedges for squeezing.