In a large pot, bring the chicken broth to a gentle boil.
Add the ginger slices, smashed garlic, and green onions to the broth and simmer for 5 minutes to infuse the flavors.
Add the sliced napa cabbage and mushrooms (if using) to the pot and cook for 5-7 minutes until the cabbage softens.
Gently add the soft tofu pieces and simmer for another 3-4 minutes to warm through.
Season the soup with light soy sauce, chicken bouillon powder or salt, and white pepper.
Turn off the heat and drizzle the sesame oil on top.
Serve hot, garnished with additional green onions if desired.