If you are planning to eat this soup on the day you make it or in the next day, add the gremolata at the time you finish making the soup. If you plan to enjoy this soup throughout the week, I recommend sprinkling the gremolata over only the amount of soup you plan to eat on that day. Then, to preserve the gremolata better, store it in a glass jar and cover with extra virgin olive oil. Store the gremolata-olive oil mixture in the fridge for a few days, and you can pour that directly on your soup when ready to reheat and eat. No need to finish the soup with extra virgin olive oil, since you’ll already have some from the gremolata oil.