Put the watermelon pieces along with curd, ginger, mint leaves in a mixie and blend it to a fine puree. Add lemon juice, salt and pepper powder. Refrigerate for at least half an hour. Pour it in a glass or bowl and garnish with few water melon pieces, sambar onion cut into small pieces and mint leaves.
Note: Sambar onion can be added raw. If you do not like the raw smell, then you can fry it in few drops of oil and add it to the soup.