Banana sorbet is one of the easiest, fail-safe desserts I know of. It has the double advantage of being a great use of bananas that have gotten too ripe to eat. Their ample starch content results in a fluffy, creamy texture.
This is a very basic recipe with only three ingredients. You could enhance it in many ways (a little orange zest, infuse a spice (star anise, cardamom) into the simple syrup, a small splash of dark rum).
*The simple syrup should be a mix of equal parts water and sugar (though often I use a little less sugar), heated until dissolved and cooled. You'll likely have some left over- I usually start with 3000g of water and 2500g of sugar.
You'll need this simple syrup, ripe bananas and the juice of 1/2 freshly squeezed lemon.
Peel the bananas and weigh them. The ratio of bananas to simple syrup is about 1:1.2 by weight. For example, if you have 4 bananas they will likely weigh about 400g. Add 480g of the simple syrup and the lemon, and puree it all together in the blender.
Don't delay: put it in your ice cream machine right away or it'll start to oxidize.