Heat the oil in a large pan over a low heat. Add the onion and leeks and cook for 10 minutes, stirring occasionally.
Add the butternut squash and cook for a further 5 minutes, then stir in the pearl barley, chicken stock and chopped tomatoes. Bring to the boil, then turn down the heat to a simmer. Cover and cook for 30 minutes.
Add the leftover pork, the broccoli and most of the chopped sage and cook for a further 5 minutes, until the pork is piping hot.
Stir in the mustard and season with freshly ground black pepper. Garnish with the reserved chopped sage and serve.