In a blender, combine mayonnaise, cilantro, lime juice, wasabi, ginger, and 1 tablespoon water. Blend until smooth, adding up to 2 tablespoons more water to thin sauce, about 3 minutes. Season with salt and pepper, and set aside.
In a large nonstick skillet, heat oil over medium-high. Season salmon with salt and pepper; place in skillet, flat side up. Cook until opaque throughout, 3 to 4 minutes per side. Transfer to a plate, and cover with aluminum foil to keep warm.
Wipe out skillet with a paper towel. Add mirin and soy sauce, and bring to a boil over medium-high. Add bok choy; season with salt and pepper. Cover, and cook until tender when pierced with the tip of a paring knife, 3 to 5 minutes; discard liquid. Drizzle salmon with wasabi sauce, and serve with bok choy.
To cook vegetables like bok choy so they're light but tasty, go beyond simply steaming: Simmer them with fat-free flavorings such as mirin and soy sauce.