To make the sauce, heat a medium pan with a little oil over a medium heat, add the onion and cook for 1-2 minutes, just to moisten the onions.
Add the tomatoes to the pan and cook for 4 minutes, stirring occasionally.
Add the garlic, chili paste (sambal), and water to the pan, then turn down to low and simmer for a further 40-60 minutes. You want to reduce the mixture right down so most of the water should absorb making the sauce fairly thick. If you do want to reduce cooking time, put only 1 cup water into the mix.
This does make a fairly big batch that can be kept in the fridge for a couple of weeks. You can also freeze the sauce too.CHEATS: Use 1.5 x 400 g/14.5 oz can diced tomatoes instead of fresh tomatoes. This will cut down your prep time.For a runnier sauce, cook for less time. If you do leave the sauce runnier and want to thicken it with a glaze, use a little arrowroot flour mixed with water and add to the mixture while warm.