Soak the dried shiitake mushrooms in a bowl of boiling water for at least 30 minutes. Once soaked, lift them out, slice thinly, and set aside for later use.

In a large pot, heat a drizzle of oil over medium heat. Add the chopped onions, minced garlic, and grated ginger. Sauté for about 5 minutes or until the onions are translucent.

Pour in the reserved mushroom soaking liquid along with the broth, soy sauce, cinnamon stick, star anise, and a pinch of pepper. Bring to a gentle boil, stirring occasionally.

Add the chicken thighs to the pot, ensuring they are fully submerged. Reduce heat to a simmer, cover, and cook for about 20 minutes.

Stir in the sliced fennel, scallion whites, and rehydrated shiitake mushrooms. Cook for an additional 5 minutes.

Introduce the chopped bok choy stems and return to a gentle simmer. Cook for about 3 minutes.

Finally, stir in the bok choy greens and bean sprouts, cooking for just 2 more minutes.

Remove the cinnamon stick and star anise. Ladle the soup into bowls, garnishing with chopped scallion greens, cilantro, and sesame oil. Serve with lime wedges.