Cook frozen udon noodles according to package directions, then drain (about 5-7 minutes).

While noodles cook, heat avocado oil in a soup pot over medium heat. Sauté chopped onion until translucent, about 3-4 minutes.

Add minced garlic and cook until fragrant, about 1 minute more.

Stir in water, bouillon paste, white miso paste, tamari, sriracha, and tahini. Whisk until the miso and tahini are fully dissolved into a smooth broth.

Add cubed tofu and a large handful of baby spinach to the broth. Cook until the spinach wilts.

Divide the drained udon noodles between two bowls. Ladle the hot miso tahini broth, spinach, and tofu over the noodles.