Soak shiitake mushrooms in 2 cups water overnight. I use a container with a lid and shake it occasionally to ensure the mushrooms are all submerged. Strain through fine mesh strainer lined with paper towels. Cut mushrooms into 1/2 inch slices. Set mushrooms and broth aside.
Soak kombu in 10 cups water for 3-6 hours
Bring kombu barely to a simmer. Let steep for ten minutes. Add bonito flakes and let simmer ten minutes. Strain through fine mesh strainer lined with paper towels. You've just made dashi.
For the Soup
Heat oil over medium heat in soup pot. Add lemongrass, ginger and garlic (if using). Sweat for 3 minutes. Add bok choy. Saute for another 3 minutes.
Add dashi and gochujang. Stir until gochujang is completely blended. Add tofu and mushrooms and bring to a simmer. Simmer for 5 minutes.