Spray your saucepan with low calorie cooking spray and place over a medium heat. Add the onions, garlic and chilli and cook gently for 5 minutes until the onions begin to soften.
Add the rest of the ingredients to the pan and stir well. Let simmer for 15 minutes, stirring occasionally to make sure it doesn't stick to the bottom.
Take the pan off the heat and let cool for 10 minutes.
If you would like a smooth consistency you can use a hand blender on the cooled sauce. Depending on how thick you would like your banana ketchup you can add a little water bit by bit and stir through to loosen it.