Fill a large pot with 2 inches of water. Add an 11-inch steamer basket to the bottom of the pot. Place salmon inside basket. Season with salt and pepper. Top salmon with strips of ginger.
Bring the water to a gentle simmer over medium-high heat. Cover pot with lid and steam for 6 minutes. Remove lid and top fish with scallion and cilantro. Replace lid and steam for 2 additional minutes.
Mix soy sauce, mirin, and 1 tablespoon of water in a small bowl while fish steams. Set aside.
Drizzle soy mixture over the fish and serve immediately.