Melt 1/2 cup sugar in an aluminium loaf tin and heat over medium heat. Continue to cook until it caramelizes, being careful to not burn the sugar. Fill a large pot of water with about 3cm of water and place over medium heat. Move the aluminium pan into the large pot and set into the water.
Combine the pumpkin pie filling, evaporated milk, condensed milk, eggs, 1/2 cup sugar, vanilla and allspice in a blender, blend until smooth. Pour the mixture into the loaf tin.
Bake in preheated oven until a knife inserted in the centre comes out clean, about 1 hour. Refrigerate for at least 3 hours, or overnight.
Run a knife along the edge of the loaf tin to loosen the flan. Invert the pan onto a serving plate and tap to release the dessert.