Preheat the oven to 180°C. Grease a large oven tray and line with baking paper.
Cut a 23cm round from the pastry, prick all over with a fork, place on prepared tray and chill.
Place pears in a bowl with the lemon juice to prevent browning.
Melt the butter in a 20cm ovenproof frypan. Sprinkle over the sugar and cook for 2 minutes or until it begins to dissolve, then stir in the honey and 1⁄2 tsp salt flakes. Arrange all but one of the pears, cut-side up, in the pan with the narrowest end pointing to the centre – you want them to fit snugly as they will shrink slightly during cooking. If there is space in the centre, cut a round from the remaining pear half and place it, cut-side up, in the space.
Cook over a medium heat for 30 minutes or until the sauce has reduced down to a thick and syrupy amber- coloured caramel. Shake the pan frequently, and spoon some caramel over the pears. Remove the pan from the heat and sprinkle over the walnuts and chocolate, fifilling in any gaps between the pears with the nuts. Cover with the pastry disc and tuck the edges into the sides of the pan with a spoon.
Bake for 25-30 minutes until puffed and golden. Set aside for 5 minutes, then carefully invert onto a plate. Serve immediately with whipped cream.