Put a 21cm-diameter frying pan with an ovenproof handle on a hob over a medium heat. Add the sugar and heat to a lovely caramel colour, stirring constantly.
Chop the butter into cubes, then add to the pan with the ginger and cinnamon, and stir to combine.
Peel and core the pears, cut into wedges, then place in the caramel and spoon over the mixture. Turn the heat down and cook for 5 to 10 minutes, or until the pears are just tender and cooked but retain their shape.
Remove from the heat and cool slightly, then arrange the pears prettily in the pan.
Roll out the pastry to about 5mm thick and cut a disc slightly bigger than your pan (about 24cm).
Carefully place the pastry disc on top of the pears and caramel, then carefully tuck it snugly around the outside of the pears and down into the sides of the pan.
Bake the pie for 30 to 40 minutes, or until the pastry is golden brown and puffed up.
Remove from the oven and leave for 10 minutes. Run a knife around the edge, place a large plate over the top and carefully turn the tart onto the plate. Serve with double cream.