Start by washing, drying, peeling the raw mangoes and slicing them. Ensure mangoes are firm and underripe, as they lend a tangy flavour to the launji.
In a kadhai, heat oil over medium heat. Once the oil is hot, add cumin, nigella, mustard, fennel, and fenugreek seeds. Let them splutter. Allow the spices to sizzle for a few seconds, releasing their aroma and flavours into the oil.
Carefully add the sliced mangoes to the kadhai along with salt and red chilli powder, stirring gently to coat them with the spices.
Add water, stir to combine, cover and let the mangoes cook until they soften slightly. This takes 15 minutes over low heat.
Add grated or chopped jaggery (or sugar) to the pan. Stir gently to dissolve it.
Reduce the heat to low and let the launji simmer gently, allowing the flavours to meld together. Stir occasionally to prevent the mixture from sticking to the bottom of the pan.
Cook until the mangoes are soft and the jaggery has dissolved, creating a thick, glossy, and slightly sticky consistency. Taste, and if required add more salt, chilli powder or jaggery.
Allow the launji to cool slightly before transferring it to a clean, airtight container. Store it in the refrigerator, and the flavours intensify over time.