Preheat the oven to 200 c.
Slice the squash in half, remove the seeds then cut lengthways into inch thick slices.
Cut the onions in quarters.
Place on a roasting tray, cover in the oil/butter, sea salt and pepper. Roast for 50 minutes until cooked through.
While this is cooking mix the tahini dressing together.
Once the veg is cooked mix with the herbs, watercress, sesame seeds and dressing.