Step 1Blanch the cabbage in lightly salted boiling water until just softened, about 3 minutes, then immediately chill in cold water. Drain well and dry the leaves with paper towels or in a salad spinner. Remove the hard core.
Step 2Cut the fish cake lengthwise into halves or thirds, making sure that there is some pink in every piece. Slice into matchsticks.
Step 3On the bottom of a cabbage leaf, place a bundle of 10 slices. Roll up the leaf. If large leaves are used, tuck in the edges as you roll, like a burrito. If small leaves are used, allow the pink part of the fish cake to show at the sides. Continue until all the leaves are filled. Dry with a paper towel after rolling. Place the rolls, leaf tip side down, on a paper towel-lined platter, cover and refrigerate.
Step 4Combine the vinegar, sugar, 1 teaspoon of salt and the monosodium glutamate in a small bowl. Refrigerate.
Step 5To serve, diagonally slice the rolls made with large leaves and stand on a serving plate cut side up. Pour the dressing over the rolls.