How to Make It
Combine 4 quarts water and kombu in a large pot. Heat over medium-high until water begins to steam and small bubbles form around kombu (do not boil), about 15 minutes. Remove from heat, and stir in bonito flakes. Let steep 30 minutes. Pour mixture through a fine wire-mesh strainer into a bowl; discard solids. Stir in tamari, and keep broth warm.
Heat 3 tablespoons grapeseed oil in a large skillet over high. Add half of mushrooms, and cook, without stirring, until mushrooms begin to brown, about 2 minutes. Stir mushrooms. Cook, without stirring, until well browned, about 3 minutes. Remove mushrooms from skillet, and repeat with 3 tablespoons grapeseed oil and remaining half of mushrooms. Remove mushrooms from skillet, and set aside. Heat remaining 1 tablespoon grapeseed oil in skillet over medium. Add onion and garlic, and cook, stirring often, until onion is very tender, about 20 minutes. Remove from heat.
Finely chop half of browned mushrooms; transfer to a medium bowl. Place remaining half of mushrooms in a food processor. Add onion mixture, and process until a paste forms, about 2 minutes. Transfer mixture to bowl with chopped mushrooms. Stir in scallions, salt, sesame oil, and pepper until combined.
Place 1 heaping teaspoon mushroom mixture in center of each of 48 wonton wrappers. Lightly brush edges of wrappers with water; fold 1 corner of each over to opposite corner, and pinch edges together to seal. Set dumplings aside.
Bring a large saucepan filled with salted water to a boil over high. Add bok choy, and cook just until tender, about 2 minutes. Using a slotted spoon, remove bok choy from water. Add one-third of dumplings to bok choy cooking water, and cook until dumplings are tender and float to surface, about 2 minutes. Remove from water, and repeat 2 times with remaining dumplings.
To serve, place 6 dumplings and 1/4 cup bok choy in each of 8 serving bowls. Top each with 1 1/2 cups kombu broth. Garnish with fresh dill, and serve with tamari and, if desired, chile oil.