In a pan over medium heat, sauté the chopped onions and mustard seeds for a minute. Add in the diced mangos too.
Top this up with all the other ingredients and turn the heat down. Mix it all together.
Let this simmer for about 35-45 minutes or until the mangos are soft, but still have a bit of structure to them.
Take this off heat, allow it to cool down completely and transfer it to a clean glass jar with a lid.
Notes
Don't have cumin powder? Simply dry roast some cumin seeds on medium heat until they get fragrant, allow them to cool down and grind them to make some homemade cumin powder.
Depending on the kind of consistency you like, you can keep the chutney chunky or mash up the mangos with a potato masher to get it to a smooth consistency.
If you want to make a healthier version of this chutney, swap the sugars in the recipe for powdered jaggery and that should work just as well.
If you don't have fresh ginger or garlic that is needed for this recipe, you can use ginger powder and garlic powder instead.
If you're worried about the chopped onions burning, you can add a dash of canola oil or any other cooking oil of your choice while sauteing them.