Heat oil in a pan & splutter mustard. Add finely chopped onion and cook till the colour slightly changes, it will take 2-3 mins. Add finely chopped beetroot, curry leaves, pepper powder & salt. Mix well. Cover & cook on low flame for 7-10 mins. Give it a stir in between.
Notes
You can add green chilly instead of pepper powder if you like. 2-3 green chilly is enough for the above recipe.