Put the mango slices in a large bowl and sprinkle with salt. Leave overnight. In the morning, drain off the juice and rinse the fruit.
Put the garlic, sugar, apples, mustard, ginger, vinegar and cayenne pepper in a pan and, over a low heat, dissolve the sugar.
Bring to the boil and add the mangoes. Simmer for 30 minutes until the chutney is thick and syrupy. Spoon into sterilised jars and seal.