Cook noodles according to package directions, omitting salt and fat. Drain well.
While noodles cook, combine soy sauce, sugar, mirin, and broth in a medium saucepan; bring to a boil. Pour into a bowl; cover and place in freezer about 8 minutes to cool slightly.
Place 3/4 cup noodles in each of 4 shallow bowls; divide tofu evenly among bowls. Ladle 1/2 cup broth into each bowl; sprinkle each serving with 1 tablespoon nori, 1 tablespoon onions, and 1/2 teaspoon sesame seeds.