Please note: I have never tried this recipe with hazelnuts. I have made it with either no nuts, or walnuts. Both methods are good. 1. In small metal cake pan, toast hazelnuts in 350F oven until fragrant, 5 to 10 minutes. Transfer to tea towel; rub off as much of the skins as possible. Coarsely chop and set aside. 2. In roasting pan, toss together brussel sprouts, onion, oil, salt and pepper. Roast in 425F oven, stirring occasionally, until tender and edges are browned - about 30 minutes. 3. Add hazelnuts, maple syrup and vinegar; toss to combine. Roast for 5 more minutes.