1. Prepare the filling: In a medium bowl, combine the finely chopped shiitake mushrooms, ground pork (if using), grated ginger, minced garlic, and the white parts of the green onions. Season lightly with salt and pepper. Mix well until everything is evenly incorporated.
2. Assemble the wontons: Place a wonton wrapper on a clean surface. Spoon about 1 teaspoon of the filling into the center. Wet the edges of the wrapper with water, then fold it in half to form a triangle. Press out any air and seal the edges firmly. For a classic look, bring the two corners of the triangle together and pinch to seal. Repeat with remaining wrappers and filling.
3. Make the broth: In a large pot, heat the sesame oil over medium heat. Add the green parts of the sliced green onions and sauté for 1 minute until fragrant. Pour in the chicken or vegetable broth and soy sauce, then bring to a simmer.
4. Cook the wontons: Carefully drop the wontons into the simmering broth. Let them cook for about 4-5 minutes, or until they float to the surface and the filling is cooked through. Gently stir occasionally to prevent sticking.
5. Add bok choy: Toss in the chopped bok choy and cook for another 2-3 minutes until tender but still crisp. If you prefer a slightly thicker broth, stir in the cornstarch slurry and cook for an additional minute until the broth thickens slightly.
6. Final seasoning: Taste the soup and adjust seasoning with salt, pepper, or a splash more soy sauce if needed. Remove from heat.
7. Serve: Ladle the soup into bowls and garnish with toasted sesame seeds if desired. Enjoy immediately for the best texture and flavor.

One tip I picked up during my culinary classes is to keep the filling moist but not too wet—this helps the wontons hold together without becoming soggy. And from my early days of cooking, I’ve learned that patience during assembly pays off. Don’t rush the sealing step; a good seal means your wontons won’t burst in the broth.