1. Heat the oil and butter in a wok or large frying pan. Add the onions and cook, stirring, for 4-5 minutes or until beginning to soften. Add sprouts and stir-fry over a medium heat for 10-12 minutes or until just tender.
2. Add the peas and cook, stirring regularly, for around 4 minutes. Add the balsamic vinegar and stir for 1-2 minutes. Serve straight away.