Soak the dried shiitake mushrooms in boiling water for 30 minutes until soft. Slice and reserve the soaking liquid.

Heat vegetable oil in a large pot over medium heat. Add chopped onion, minced garlic, and grated ginger; sauté for 5 minutes until fragrant.

Pour in the reserved mushroom soaking liquid, broth, soy sauce, cinnamon stick, star anise, and black pepper. Bring to a boil, then reduce to a simmer.

Stir in the chicken thighs, ensuring they are submerged. Let simmer uncovered for 20 minutes until chicken is cooked through.

Add chopped fennel, white parts of scallions, and reserved mushrooms. Cook for 5 minutes until fennel softens, then add bok choy stems and simmer for 3 minutes.

Stir in bok choy greens and bean sprouts, cooking for another 2 minutes until greens are wilted.

Remove cinnamon stick and star anise. Ladle soup into bowls and garnish with green scallions, fresh cilantro, and a drizzle of sesame oil. Serve with lime wedges.