Whisk together the teriyaki sauce, cornstarch, brown sugar, ginger and garlic in a pot, then set aside.
Cut the tofu in half lengthwise. With a small spoon, create a shallow well for the filling by carefully scooping out a little of the tofu from the top of each half.
Line a large bamboo or stainless steamer with perforated parchment paper. Carefully set in the tofu halves. Arrange and mound the prawns, snow peas, bell pepper and the tofu you scooped out, on top of the tofu halves.
Cover and steam the tofu over simmering water 5 minutes, or until the prawns are cooked. While this occurs, bring the teriyaki sauce mixture to a simmer and cook until sauce lightly thickens.
Carefully lift the tofu out of the steamer and set on plates. Spoon over the sauce and serve.