Preheat the oven to 350º F (175º C)
Slice the bananas in bias or diagonal cuts. You can also cut them in rounds, but the bias or diagonal cuts offer more surface to caramelize. However, it's up to you. Place the banana slices in a bowl.
Squeeze a lemon and pour the juice onto the banana slices. Gently make sure all slices are coated with juice. The lemon juice will help prevent the bananas turning brown. It will also enhance the flavors of the bananas. Add the vanilla and blend with the bananas.
In a frying pan over medium heat melt the butter. Add the sugar and continue to heat until it dissolves. Continue over heat until the butter and sugar begin to caramelize. Don't overheat, as the buttery caramel sauce will cook more during the baking process in the oven.
Reduce the heat to low. Add the banana slices to the frying pan, without crowding too much. If you need to, you can slightly cook the banana slices in two batches. Cook only until the banana slices to start turning soft, without cooking them through. It's important that the slices remain in tact.
In a 9 inch (23 cm) pie dish, arrange the bananas in concentric circles, making sure the entire surface is covered. Once the first layer is finished, use the remaining bananas to add a second layer of concentric circles. If you have extra slices of bananas, add them on top.
Pour all the caramel sauce over the banana slices.
Roll out the pastry crust dough on a floured surface. Fold the dough over the rolling pin and then gently lay it on top of the pie dish with the bananas. Tuck the edges of the dough along the inside edges of the pie dish.
Place the pie dish into the preheated oven. Bake for thirty minutes or until the crust becomes golden brown.
Remove the pie from the oven and let it cool a little so the caramel starts to solidify a bit. Otherwise, the caramel will spill everywhere when you flip the pie over. When it's ready to flip, place a serving dish on top of the pie dish, pass a knife along the edges, and then flip it over so the crust lies on the bottom and the fruit on top. Serve warm. Reheat later if needed.