Thoroughly rinse the fish inside and out and pat dry with paper towels. Makes 3 to 4 shallow slashes on each side of the fish.
In large bowl, combine the soy sauce, ginger, sesame oil, garlic and red pepper flakes. Stir to mix well. Drizzle about 1 tablespoon of the mixture over each side of the tilapia and use your hands to rub the sauce evenly over the fish and into the slashes.
Add the cabbage and green onions to the remaining sauce and toss to coat. Arrange the cabbage on a heat-proof plate that will fit inside your steamer and set the tilapia on the bed of cabbage. Put the plate in the top portion of a steamer; if the fish is a little too large for the steamer, trim the tail.
Bring about 3 inches of water to a boil in the bottom of the steamer. When the water boils, set the steamer basket above and cover with the lid. Steam until the cabbage is tender and the fish is just opaque through the thickest part, 15 to 20 minutes. Transfer the fish and cabbage to a serving platter, sprinkle the toasted sesame seeds over and serve.