Remove stems and slit the green chillies. Put them in a big bowl.
Add mustard powder, crushed mustard, turmeric powder, red chilli powder, sugar, salt and vinegar. Mix well.
Take a sterilized jar. Transfer the chillies into it along with the masala. Top it up with oil. Put the lid on and shake so that the oil gets mixed with the chillies.
Let it mature for at least two weeks. Mix a few times while the pickle is maturing.