Step 1In a medium stockpot, combine 8 cups of water with the konbu and set the mixture aside to steep for 30 minutes. Place the stockpot over medium heat and bring it to a boil. Remove and discard the konbu, and stir in the remaining 2 tablespoons of water. Add the bonito and stir it once to combine. As soon as the liquid boils again, reduce the heat to low and simmer for 5 minutes. Remove any scum that appears on the surface, as this can adversely affect the flavor.
Step 2Remove from the heat and set aside to steep for 15 minutes. Strain the liquid through a fine sieve or cheesecloth; don't squeeze the bonito flakes. Discard the bonito flakes. This should make 4 to 5 cups dashi.
Step 1To prepare the broth, in a medium bowl, combine the dashi, mirin and soy sauce.
Step 2In a pot (such as a Japanese clay donabe or cast-iron Dutch oven), place the negi, then pour in the broth.
Step 3Cover the pot and bring it to a boil over high heat. Decrease the heat to a simmer, uncover the pot and add the pork belly, arranging the slices on top of the negi. When the hot pot returns to a simmer, continue to cook for 3 minutes. Add the spinach, mizuna, shungiku leaves, watercress and baby bok choy in a random pile on top of the pork belly. Cover and simmer until the greens are cooked through, 2 to 3 minutes; gently press the greens into the broth if necessary for even cooking.
Step 4Transfer the hot pot to the dining table. Serve the ingredients together with the broth in small bowls, garnishing with the white pepper.
Step 5Alternatively, this can be cooked tableside with a portable gas burner: Arrange the ingredients on serving platters. After preparing the broth, do all the cooking at the dining table. Add the negi all at once. Then cook a little of the greens and pork belly at a time.