Soak red chilli & tamarind in water for 30 min. Grind onion, red chilli, tamarind, salt in the blender with just enough water to make paste.
COOKING:
In a pan pour oil. When hot, add mustard, split urad dhal and curry leaves. When mustard splutters add the ground paste. Allow it to boil and add oil little in middle if necessary. Stir often to avoid burning it. When the oil segregates take off from the Stove.
TIPS:
Use sesame oil for better taste.
Chutney is very spicy, reduce the number of red chilies to match ur taste.
Recipe by Sashi Recipes at https://www.sashirecipes.com/onion-chutney-vengaya-chutney.html