This could not be simpler. But it *is* a very tasty sauce - you can use it as a dipping sauce for veggies or chicken (especially good with Coconut Chicken Nuggets) or stir it into yogurt, or use it in a dessert, like this Fruity Semifreddo.
You need one banana, and about 200 ml of mango, diced. And a little bit of lime. Mix, mash or blend, into a smooth sauce. And that's that. It keeps for a few days in the fridge, but it'll get darker over time.