Heat 2 teaspoons of sesame oil in a large saucepan over medium-high heat. Add the leeks and sauté for 2-3 minutes, until lightly browned.
Add 1 quart (4 cups) of water to the pot, along with the tomatoes, vinegar, soy sauce, fish sauce, ginger paste, and sriracha (if using).
Allow the mixture to come to a boil, then turn off the heat. Garnish with fresh cilantro. Serve hot.
Notes
To store your Thai Tomato Soup, keep it in an airtight container in the fridge for up to 4 days. You can also freeze it in a freezer-safe container for up to 3 months.To reheat, thaw overnight in the fridge, then warm on the stovetop over medium heat or microwave in 30-second intervals, stirring occasionally, until hot.