Preheat the oven to 200C and bring a pan of water to the boil. Spread hazelnuts on a baking tray and toast in the oven for roughly 6 minutes until golden brown. Chop roughly.
Prepare the sprout tops by tearing them into strips and dropping into the just boiled, now simmering water until just tender (roughly 2-3 minutes). Drain well and return to the pan.
Melt the butter and heat until it starts to brown, top over the cooked sprout tops, add the hazelnuts, lemon juice and salt and pepper. Toss and serve.