This recipe is one of those happy accidents that results from a combination of two things: a half-empty fridge and a total lack of desire to go grocery shopping. I had a whole bunch of carrots and sweet potatoes (don't ask me why) and almost nothing else to work with - and it turns out that's really all you need!
If you keep a well-stocked Asian pantry, this soup is totally dirt-cheap and easy to put together. If not, I would highly recommend investing in all the Asian ingredients that are called for (including the garnishes) because they are great staples, are found in lots of different Asian dishes, and make this soup incredibly flavorful and unique. The tahini not only adds good nutrition (sesame seeds are packed with good fats and protein!) but also gives the soup a thicker, creamier consistency. The soy sauce adds that wonderful, mysterious umami flavor and a complex saltiness. The drizzle of toasted sesame oil really brings out the pure flavor of sesame and adds a toasty nuttiness to the dish. I used black sesame seeds as a finishing touch because they added one more layer of sesame flavor and they look really beautiful sprinkled on the soup, but blanched sesame seeds or toasted white sesame seeds would work just fine. Lastly, Sriracha is a spicy, garlicky and totally multi-purpose condiment that adds a nice heat and extra depth of flavor to the soup. I used about a tablespoon per serving for a medium-strength heat. Use more Sriracha if you like your soup really spicy.
In a large pot over medium heat, saute the onions in olive oil until translucent, about 8 minutes. Add potatoes, carrots and ginger and cook until ginger is fragrant, about two minutes. Deglaze the pan with mirin, then add stock. Raise the heat to high and bring liquid to a boil. Reduce heat to low, cover pot and allow to simmer, stirring occasionally until potatoes and carrots are very tender, about 40 minutes.
In a small bowl or measuring cup, add a little of the hot liquid to the tahini and stir to dissolve. Add mixture to soup and puree with an immersion blender until smooth and creamy. Serve with a sprinkling of sesame seeds and a drizzle of oil and sriracha sauce to garnish.