Heat the olive oil in a large frying pan over a medium-low heat. Add the onions and peppers, and cook for 20 minutes, stirring occasionally until tender and caramelised.
Meanwhile, in a small frying pan, toast the hazelnuts over a medium heat for 4-5 minutes,stirring frequently until just golden. Tip the nuts on to a chopping board and roughly chop.
To make the dressing, whisk the vinegar, mustard and honey together and season with freshly ground black pepper.
Divide the salad leaves between 8 small plates. Top with the caramelised onions and peppers, then scatter over the crumbled goat's cheese and toasted hazelnuts. Drizzle with the dressing and serve immediately.