Prepare the Vegetables:
Separate the bok choy leaves and wash them thoroughly, as mud often hides between the stems. Slice the bok choy leaves and set aside. Slice the green onions, keeping the white bulbs and green stalks separate. The green stalks will be used as a garnish.
Sauté the Aromatics:
Heat the oil in a pan over medium heat. Add the sliced green onion bulbs and sauté until softened. Reduce the heat slightly and add the minced ginger and garlic. Stir-fry for 1-2 minutes, being cautious if using paste, as it may splatter.
Cook the Bok Choy:
Add the sliced bok choy to the pan and cook for about a minute, allowing it to wilt slightly.
Add Spices and Broth:
Add the star anise, bay leaf, and cinnamon stick to the pan. Pour the water mixed with concentrated stock over the bok choy and spices. Stir in the soy sauce and rice vinegar.
Simmer the Soup:
Bring the soup to a boil, then reduce the heat and let it simmer for 8-10 minutes, stirring occasionally.
Finish the Soup:
Remove the soup from heat and stir in the lime juice. Mix well to combine all the flavors.
Serve:
Carefully remove the whole spices (cinnamon stick, bay leaf, and star anise) from the soup and discard them. Ladle the hot soup into bowls and garnish with sliced green onion stalks, red pepper flakes, and a sprinkle of sesame seeds or a drizzle of sesame oil if desired.