Heat a large heavy-bottomed pot over medium heat and add the oil.
Add the ginger, garlic, and red chili. Cook for about 1 minute, stirring continuously, until fragrant. Keep the heat moderate so the garlic does not brown.
Stir in the nigella seeds, coriander, cumin, turmeric, cardamom, cloves, cinnamon, and salt. Let the spices bloom for about 30 seconds, stirring constantly.
Add the diced mangoes, sugar, and vinegar. Stir thoroughly to combine and dissolve the sugar. Increase the heat and bring the mixture to a rolling boil.
Reduce to medium-low and simmer uncovered for 50-60 minutes, stirring occasionally. As the chutney thickens, stir more frequently to prevent sticking.
Cook until the mango is completely tender, and the mixture reduces to a thick, glossy consistency that coats a spoon. Lightly mash some of the mango pieces if you prefer a softer texture, keeping some chunks for body.
Remove from heat and allow to cool completely before transferring to jars.