Toss mango cubes with salt, spread on a tray, and sun-dry for 4 to 6 hours until slightly wrinkled.

Dry roast mustard, fenugreek, and fennel seeds over medium heat until fragrant, about 2 minutes. Cool completely, then coarsely grind with nigella seeds.

Mix ground spices, turmeric, chili powder, asafoetida, and mustard oil into a red paste.

Toss dried mango pieces in the masala until well coated, then pack tightly into a clean glass jar.

Seal the jar loosely and place in direct sun for 4 days, shaking once daily. The oil should darken and the mangoes appear plump.

Store in a cool dark place. Spoon out with a clean dry spoon and serve at room temperature.