For mustard oil: In a small skillet, toast mustard seeds over medium heat for 1 to 2 min. or until fragrant. Transfer immediately to a small bowl to cool. Slightly crush seeds in a spice grinder or mortar and pestle. Transfer to a small saucepan and add oil. Heat over low for about 5 min. Once seeds have slightly darkened and the oil has a nutty smell, remove from heat and let cool slightly. Strain oil into a clean container and discard seeds. Let cool and store in an airtight container in the refrigerator. (You’ll have more than you need for the chutney recipe. Use remaining for salad dressings, aioli, low-heat cooking oil, or finishing oil.)
For chutney: In a medium skillet, toast fenugreek seeds over medium heat for 1 to 2 min. or until fragrant. Transfer immediately to a spice grinder or mortar and pestle and grind into a coarse consistency; set aside. In same skillet, toast charnushka and chiles for 1 to 2 min. Transfer to a plate to cool. Add oil, mangoes and garlic to the skillet and cook until fragrant, about 5 min. Add toasted charnushka and chilies and cook a few more minutes. Stir in mustard oil, brown sugar, and reserved ground fenugreek. Simmer over low heat for 30 min., stirring occasionally. Sugar will dissolve and mixture should become thick and sticky. Remove from heat and let cool to room temperature. Store in an airtight container in the refrigerator. Use within a couple of weeks.