1. Preheat the oven to 190°C, gas mark 5. For the hazelnut dressing, roast the nuts on a baking tray until golden – about 6-8 minutes. Pulse in a food processor with the oils, vinegar and a few pinches of salt until very roughly chopped; set aside.
2. Heat the olive oil in a lidded frying pan or large saucepan over a medium heat. Halve and slice the onion into 1cm slices; fry for 10 minutes until it starts to colour. Season the mackerel and place on top of the onions. Pour over the vinegar and then immediately cover with a lid and take off the heat, allowing the fish to steam.
3. After 5 minutes, toss the onions and mackerel together, loosely breaking up the fish. Transfer to a large bowl with the apple, celeriac and parsley. Pour over the lemon juice and dressing, check the seasoning and serve.