Clean and dry your raw mangoes thoroughly - you want to make sure there is no sap/debris. Peel and cube them into bite sized pieces
Add salt to the mangoes and toss to marinate. Set them aside for 30 minutes. This helps draw out excess moisture as well as helps preserve. Drain the water released keep them as dry as possible.
Toast your fenugreek seeds until fragrant. In a mortar pestle, coarsely crush the fenugreek and mustard seeds. Add to the mangoes along with chilli and turmeric powder.
Heat your oil until it starts to shimmer. Add to the mango mixture and mix well until combined. Once it has cooled down, taste and adjust salt and a touch of sugar.
Store refrigerated in a dry container. Use a clean dry spoon every time you eat some of it. Tastes best as a condiment to Indian meals.
Notes
Peeling the mangoes is optional - you can choose to make the pickle with the skin on.
For Indian style pickles I personally love using sesame or mustard oil - their pungent fragrance is part of the flavour profile. Having said that, feel free to use any neutral oil ( like canola/vegetable).
You can use kashmiri chilli powder if you prefer your pickle milder. taste and adjust - start with 1tsp and see how you like it.