To prepare fish: Bring 1 to 2 inches of water to a boil in a pot large enough to hold a two-tier bamboo steamer. (If you don't have a steamer, improvise by setting mugs upside down in a large pot and resting a large heatproof plate on top.) Put a heatproof plate in each of the steamer baskets. Place 3 portions of fish on each plate with a slice of fresh ginger on top. Stack the baskets, cover and set over the boiling water. Steam the fish for 7 minutes per inch of thickness.

To prepare sauce: Meanwhile, combine minced ginger, garlic and sesame seeds in a small bowl. Heat grapeseed (or canola) oil in a medium skillet over medium-high heat. Add the ginger mixture and cook, stirring, until fragrant, 1 minute. Add sesame oil; allow the mixture to get hot. Add soy sauce (be careful, it will splatter a bit) and cook for 1 minute more.

Transfer the fish to a deep platter. Discard the ginger slices. Pour the sauce over the fish and garnish with scallions.