Peel and slice the Mango into small Cubes and place them into a small pot (see note 1 if using frozen mango).
Add the Sugar and Lime Juice, and stir to combine.
Cook on low to medium heat for about 10 to 15 minutes (see note 2) or until the fruit is very soft and starts to fall apart, occasionally stirring to avoid burning. Remove from the stove and leave to cool down for about 5 minutes.
Using an Immersion Blender, blend the cooked fruits until very smooth. It will take a few minutes of blending to get a very smooth, thick sauce.
Once blended, taste the coulis and adjust the sweetness / acidity with a little bit more sugar or lime juice if needed
Optional: to get a very smooth and thiner sauce, strain the coulis through a mesh sieve to remove any fibers or unblended fruit
Leave to cool down completely then transfer to a sealed jar / container and keep in the fridge for up to a week.