Cook the onions in boiling water for 15 minutes, until they are just tender. Drain and allow to cool until they can be handled.
Mix the apples with the breadcrumbs and cheese, then stir in the chopped watercress.
Remove the middle of each onion – this is easiest if you gradually scoop out the layers of onion with a teaspoon. Leave an unbroken shell about 2 layers thick. Chop the scooped-out onion from 1 of the onions (discarding the rest) and add to the cheese mixture. Mix well and season to taste, then press the mixture into the onion shells.
Stand the onions in an ovenproof dish and dot them with the butter. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 35–40 minutes, or until the stuffing is cooked through.