Peel and dice 1.35kg of firm unripe mangoes. Toss with 3 tablespoons of kosher salt, cover, and refrigerate for 24 hours.
Heat 3 tablespoons of oil in a saucepan over medium-low heat. Add 2 teaspoons of mustard seeds, then 6 chopped garlic cloves and 1 medium chopped Fresno chili. Cook for 2-3 minutes without browning.
Stir in 1 tablespoon of turmeric, 2 teaspoons of ground fenugreek, 2 teaspoons of ground coriander, 2 teaspoons of ground cumin, and 1 pinch of cayenne pepper, and cook for 1 minute.
Add the mangoes, 3 tablespoons of brown sugar, and 1 cup of water. Simmer for 5-6 minutes, until the mango softens. Off the heat, stir in ½ cup of white vinegar and adjust the seasoning.
Purée with an immersion blender to the texture you like, and let cool.
