Heat 2 tablespoons olive oil in a large saucepan over medium heat and cook onion until soft and translucent, about 5 minutes. Add tomatoes and cook for 2 minutes, stirring constantly.

Pour water into the saucepan and bring to a boil. Add 4 sprigs of mint and zucchini. Reduce heat to low and simmer soup, covered, for 15 minutes. Remove mint.

Stir together cornstarch and cold water and add to the soup. Increase heat to medium and whisk cornstarch mixture so soup thickens. Season with salt and pepper. Remove from heat and set aside to cool.