Boil the sprouts for 3-4 minutes, until tender but firm. Drain and run under cold water, then place on a clean tea towel to absorb the excess water.
Meanwhile, melt the butter and olive oil in a large, heavy-based frying pan. Add the shallots and gently cook for 5 minutes, then add the pancetta and thyme.
Cook for a further 10 minutes, until beginning to crisp. Add the hazelnuts and sprouts to the pan, fry over a high heat for 2-3 minutes to heat through, then serve.