Preheat oven to 180C. Place hazelnuts on a baking tray and roast until golden (5-6 minutes). Rub with a tea towel to remove skins, coarsely chop and set aside.
For sorrel butter, combine ingredients in a food processor and pulse until well combined and sorrel is finely chopped. Season to taste and set aside.
Heat butter in a large frying pan over high heat. Add trout fillets, flesh-side down, and cook, turning once, until golden and skin is crisp (5-6 minutes). Season to taste and serve warm, scattered with hazelnuts, with a dollop of sorrel butter.
Note Sorrel is a leafy green vegetable similar
in appearance to spinach, but with a sharp acidic lemon flavour.
It's available from specialist greengrocers.