Green Sauce detail is missing?

I had some homemade pesto on top of the soup! You could also use herb oil instead :)

The herbal garnish I'm the photo certainly looks like basil, so you could use that.
But since the author omitted referring to the green garnish in the entire recipe (headnote, ingredients, directions), I'd say you have free choice here:
1) omit the green drizzle entirely if you don't like it or lack the ingredients;
2) use any single green herb you like or have that goes with tomatoes. Ditto for the liquid to make the herb into a drizzle.