Preheat oven to 180°C.
Roast hazelnuts and remove their skins. The easiest way to do this is by rubbing them in a clean tea towel on the kitchen bench, then you can easily pick out the skinless nuts. Roughly chop.
Remove any brown outer leaves from the brussels sprouts. Toss with oil, salt and pepper and place on a roasting tray. Cook in oven until softened in the centre and the edges turn crispy brown, occasionally shaking the roasting pan.
Make a dressing from the oil, mustard, onion and lemon juice, then season to taste.
Just before serving, toss roasted sprouts and hazelnuts with the dressing in a large pot and warm through. Serve immediately.