Cut the cheeks off each mango and chop the flesh into large chunks. Also salvage any mango flesh you can from around the seed.
De-seed the chilli and chop into a few large pieces. If you want the sauce to be hotter, leave the seeds in.
Peel the onion and cut into quarters.
Add all ingredients to a blender and blend until smooth.
Pour the sauce into a large saucepan and bring to a boil. Reduce the heat and simmer for 10 minutes.
Cool for 5 - 10 minutes then transfer to a large jar or bottles. Store in the fridge where it will keep for a couple of months.
Video
Notes
Note - This sauce will thicken up a little in the fridge however if you would prefer it much thicker simply add 2 tsp of arrowroot or corn-starch to a little water to make a slurry, add it to the hot sauce and simmer until it thickens. This should only take a few minutes.