Put the butter and sugar in a saucepan and heat gently until the sugar dissolves. Shake the pan every now and then but don't stir it. When the sugar has melted turn the heat up a little and let it bubble until it turns a caramel colour. Don't worry if the butter and sugar seem to separate, it'll still be fine.
Cut the pears into quarters. Cut out the core and then slice each quarter into two or three pieces, depending on how chunky you want it to be.
Pour the caramel into the bottom of an ovenproof dish. Arrange the pear slices on top. Top with puff pastry - the same size as the dish you are using. You may not need all the puff pastry. Make a couple of slits in the pastry so steam can escape.
Bake for 35 minutes in a 200°c oven.
Let it cool for about 10 minutes after removing from the oven. Then put an upside down plate on top and flip the dish and plate over to remove it.