1. Chip clams for 1 hour (see Notes). Remove clams from water and keep in refrigerator.

2. Light a gas burner or heat an electric burner until hot. Lightly grasp a nori sheet with tongs and pass quickly several times through flame or over electric burner, stopping when nori gives off a faint toasted seaweed odor. The nori should not change in form or color. Coarsely chop nori and set aside.

3. Heat a sauté pan over medium-low heat and add sesame seeds. Toast seeds, shaking pan frequently, just until fragrant and golden brown, 3 to 5 minutes. In a spice mill or mortar with pestle, grind nori and sesame seeds together until crushed fine. Transfer to a small bowl and stir in chopped dark green scallions.

4. In a stockpot, combine soy sauce, vinegar, mirin, and ¼ cup water. Bring to a simmer and add 2-inch white and pale green scallion pieces. Cook until crisp-tender, about 3 minutes. Add clams to stockpot, cover, and cook just until all the clam shells have opened, about 8 minutes. Stir midway through so that clams buried at the bottom get an equal shot of steam. Discard any clams that do not open.

5. Put 6 clams in each of 4 shallow bowls. Distribute scallions and ladle cooking broth into the bowls. Dash a few drops of olive oil on the surface of the broth. Sprinkle with sesame-nori-scallion mixture. Serve hot.