Pan roast the mustard seeds, fenugreek seeds, garlic and red chilies in a bit of oil over medium heat.
Once they turn fragrant, take them off heat, let them cool down and then transfer them to a blender jar. Add the cumin powder and blitz.
You should get a chunky consistency like this. Carefully remove this spice paste.
Transfer it to a blender jar with the chopped mangos and the other ingredients and blend until smooth.
Notes
If the sauce has turned too thin, you can add a bit of cornflour to it, give it a quick mix and let it simmer for a couple of minutes on low heat until it thickens up.
If the sauce seems too thick, add some more lemon juice or a splash of water to get it to the right consistency.
For better, intense flavor, you can choose to cook down this sauce in a small pan over low heat, stirring continuously for about 5-7 minutes.
To lend the sauce that authentic Middle Eastern flavor, you can also add a pinch or two of Sumac powder if you have it at hand.