Place cubed mango in a bowl or dish, dry mango cubes with paper towel ensure there is no moisture on them. Sprinkle kosher salt and turmeric over mango. Stir to distribute seasoning. Allow to marinate overnight or at least 8 hours.
The salt will release moisture from the mangos, drain the water. Add the achar masala to the mangos, stir to combine and marinate for at least 4 hours, up to overnight.
Heat oil in a small saucepan, add fenugreek seeds. Heat until oil simmers. Remove from heat and allow to cool completely.
Transfer seasoned mango cubes to an airtight jar, pour cooled oil over the mango cubes. Stir to distribute the oil.
Spicy mango pickle can be eaten right away, but it is best after at least 3 days of marination.