Add the sweetener, cayenne, cumin and salt and taste. You want a good mix of hot, salty, tart, and sweet. Since mangos vary in their tartness, adjust as needed.
In a small saucepan or heatproof ladle, heat the oil and when it is hot and starts to shimmer, add in the mustard seeds and let them pop. This classic tempering technique allows the mustard seeds to bloom in flavor and also flavors the oil.
Once the seeds have sputtered for a bit (about 10 seconds), pour the flavored oil onto the chutney and mix well.
Let the flavors marinate for about 10-15 minutes and serve with Indian food, fish, or grilled meat.
This would also be a great dip for sweet potato fries.