Pre-heat your oven to 200C Fan/220C/425F/Gas Mark 7
Roll out your puff pastry sheet onto a baking tray lined with baking paper. Score a 1cm border around the edges using a sharp knife, being careful not to cut all the way through the pastry
Using a fork, prick the centre of the pastry all over. This will stop it from rising as much as the edges. If you like, glaze the border with milk or beaten egg
Bake the puff pastry sheet for 15-20 minutes, or until risen and golden
While it is baking, melt the sugar in a frying pan on a medium heat. It will turn into a golden brown caramel
Add the banana slices and flip them over to coat both sides with the caramel. Move the banana slices to a non-stick surface like a silicone mat or baking paper
Remove the pastry from the oven, the centre will have risen somewhat, so use the back of a spoon to press it back down
Add the Nutella in spoonfuls all over the centre of the pastry while it is still warm. After a minute, the Nutella will start to melt, use a palette knife to spread it out and cover the tart
Add the bananas all over, then add spoonfuls of the marshmallow fluff
If you like, use a kitchen blowtorch to brown the marshmallow fluff. Or you can pop it under the grill - but watch it very closely!
Sprinkle the chopped roasted hazelnuts all over
Serve immediately. Leftovers can be kept in an airtight container in the fridge for 2 days. You can reheat slices of the tart in the oven for 5 minutes at the same temperature it was baked on, or in the microwave on high for 60-90 seconds