Start by cooking frozen udon noodles according to package directions, then draining the noodles. If you aren't using frozen udon noodles, cook your preferred type of noodle. You will need 1 cooked cup of pasta in total.
While the noodles are cooking, bring a soup pot to a medium heat with oil. Saute onions and garlic until translucent and fragrant. This will take a few minutes.
Add water, Better Than Bouillon no chicken base, miso paste, tamari, sriracha, and tahini to the pot. Stir until everything is evenly combined. You want the miso paste and tahini to dissolve into the broth.
Add cubed tofu and a big handful of baby spinach to the tahini broth. Allow the spinach to wilt in the soup. (Or if you'd prefer, you can add the spinach later to the individual serving bowls as a topping.)
Put one half of the drained noodles in a soup bowl and one half in the other bowl. Top the noodles with miso tahini broth, spinach, and tofu.
Finish the bowls of soup with a drizzle of sesame oil. It's optional, but it really brings out the "sesame-ness" of the tahini. Not too much sesame oil is needed. About ⅛ to ¼ teaspoon of sesame oil per bowl is plenty.