How to Make It
In a medium saucepan, bring the broth, water, mirin, soy sauce, vinegar, ginger, garlic and sugar to a boil.
Meanwhile, bring a large pot of water to a boil. In a bowl, soak the rice noodles in very hot water until pliable, about 10 minutes. Drain the noodles and add them to the boiling water. Cook until tender, about 4 minutes. Drain and transfer to 6 shallow bowls.
In a large nonstick skillet, heat the oil until shimmering. Season the salmon fillets with salt, add them to the skillet skin side down and cook over high heat until browned, about 4 minutes. Turn and cook until medium-rare, about 4 minutes longer. Set the fillets on the noodles.
Add the bok choy to the boiling broth and cook until bright green, about 2 minutes. Using a slotted spoon, divide the greens among the bowls and spoon in the broth. Garnish with the scallions and serve.