Combine the coconut milk and onion in a bowl; set aside.
Bring a large pot of water to a boil over medium heat. Boil the bananas in their skin for 30 to 40 minutes; drain. Run the bananas under cold water until they are cool enough to handle. Remove the bananas whole from their skins.
Heat a large saucepan over medium-high heat. Place the bananas in the pan. Pour the coconut milk mixture over the bananas. Cook until the coconut milk is foamy. Allow to cool 5 minutes before serving.
