In a bowl, cover the glass noodles with very hot water and let soak. Season the fish with salt and pepper. Peel and grate the ginger. Trim the root end of the scallion and rough chop the scallions. In a bowl, combine the miso with 2½ cups water and whisk until dissolved. Peel and mince the garlic. Wash the tatsoi but don’t dry it.

In a 12-inch frying pan, heat 2 tablespoons oil over medium high heat until hot but not smoking. Put the scallions in a small heatproof bowl with the scallion seasoning. Pour the hot oil over the scallions. The oil should sizzle. If it doesn’t, it isn’t hot enough. Stir to dissolve the seasoning. Let stand until ready to use.

In a 2-quart saucepan, heat 1 tablespoon oil over medium heat until hot but not smoking. Add the ginger and cook just until fragrant. Add the miso-water mixture, bring to a boil, reduce to a simmer, and cook until the miso is dissolved and the broth is infused with the flavor of the ginger, about 3 minutes.

Add 2 more tablespoons oil to the pan and warm over medium heat until hot but not smoking. Pat the salmon dry with a paper towel and generously season with salt. Add the fish to the hot pan, skin side down, and cook until the skin is crisp, about 5 minutes. Turn and continue cooking until the flesh is opaque, about 3 minutes longer. Transfer the salmon to a plate. Add the garlic and cook just until fragrant, about 30 seconds. Add the tatsoi, season with salt, and cook just until wilted, about 1 minute.

With a pair of kitchen scissors, cut the noodles two or three times. Drain, and add to the miso broth. Divide the noodles between serving bowls. Top with the fish and the tatsoi and ladle the broth on top. Add the scallion oil seasoning. Serve hot.