Preheat the oven to 160°C.
Place your pastry sheet in a tart tin with its baking paper. Using a fork, pierce the pastry and bake it blind for 10 minutes.
In a bowl, mash 2 bananas with the lemon juice and slice the remaining bananas into very fine rounds.
Remove the puff pastry from the oven and spread your banana puree over it. Then place the banana rounds on top. Sprinkle with cinnamon.
Put into the oven for 10 minutes.
Leave to cool before serving.