Add the minced garlic, mushrooms, white parts of the green onion, and chicken thigh pieces to a pot. Pour the chicken broth into the same pot and bring everything to a boil. Then reduce to a simmer.
Once the soup is simmering, add the gochugaru, gochujang, soy sauce, and sugar (optional). Cover the pot and let everything simmer for 8-10 minutes. Then add the remaining green onions.
Optional: If you want to use more vegetables, add them according to their cook time to the soup.
In the meantime, cook the udon noodles in a separate pot according to package instructions.
Transfer the noodles to a large bowl. Then pour the broth and other ingredients over the noodles. Enjoy!
