Place the pumpkin into a roasting tray. Drizzle over the vegetable oil, sprinkle over the nigella seeds, then season, to taste, with salt and freshly ground black pepper. Roast for 20-25 minutes, or until softened.
Meanwhile, heat the honey and cider vinegar in a small frying pan over a medium heat, or until dissolved. Stir in the walnuts until well combined.
Mix together the watercress leaves, shallot, roasted pumpkin and pumpkin seeds until well combined. Drizzle over the honey and vinegar mixture and stir until the leaves are coated in the dressing. Sprinkle over the crumbled feta.