Finely chop the shallot and garlic. Peel the mangoes and dice the flesh into small pieces.
Heat a drop of oil in a small, heavy-based saucepan over a low heat and add the cumin, black cumin and mustard seeds. Toast gently until fragrant and the seeds begin to pop.
Add the chopped shallot, garlic and salt. Stir for a few seconds, then pour in the vinegar. Stir for a minute or so.
Add the diced mango and muscovado sugar. Cook gently for about 10 minutes, stirring occasionally, until the mango softens and the chutney thickens.
Spoon the hot chutney into a clean jar and screw the lid on tightly. Allow to cool completely before storing or serving
Keeps well in a sealed jar in a cool place; refrigerate after opening. Best enjoyed within 2–3 weeks.