Preheat oven to 180C. Peel sweet potato, cut in half lengthwise and then cut into 2cm thick slices.
Peel onion and cut into thick wedges.
Place vegetables on an oven tray and drizzle with a little olive oil. Roast for about 40 minutes until cooked.
Place hazelnuts on an oven tray and roast for about 5 minutes to loosen skins.
Remove from oven and rub off skins in a clean tea towel. Set aside.
Wash watercress and pick through to remove large stalks.
For the dressing
Whisk all the ingredients together.
Place watercress in large salad bowl, add roasted vegetables and hazelnuts and toss with the dressing. Cut the blue cheese into bite-sized pieces and scatter over the salad. Serve immediately.