Lightly steam the potato slices. You can use new potatoes or any potato that doesn't have a high starch content works better than the Idaho potatoes. Set aside.
Thinly slice three of the carrots into coin shapes and lightly steam. Set aside.
With the remaiing 3 carrots, cut groves into each carrot, lengthwise, then thinly slice them like you did with the previous carrots, buts you will have flower shapes because of the groves which will form petals. Lightly steam and set aside.
Take out the cabbages. Carefully remove 20 large leaves and blanch them in boiling water in batches, 2-3 minutes per batch. Let cool.
Cut out the tough stem (vein) from each cabbage leaf. Set aside.
Grate the remaining cabbage and lightly steam and set aside.
In a large frying pan put the oil and onion and stir-fry until the onion is tender. Add the corned beef, 1/4 cup water and salt and pepper as needed. Check to make sure that the mixture doesnt get too dry or you may have to add a little more water. Mix well, mashing the meat into small pieces with the mixing spoon.
Take out a cabbage leaf. Put inside the leaf 4-5 potato slices(about 1/20 of the potato slices), 3-5 carrot rounds, some of the meat mixture and a little of the grated cabbage. Roll up the cabbage leaf and place it in a lightly oiled lasagna pan.
Fill the remaining cabbage leaves the same way.
Pour the vegetable broth over the cabbage leaves. Sprinkle with salt and pepper if desired. Sprinkle the carrot flowers on top. Cover securely with foil.
Bake 350 degrees for at least 45 minutes or until everything is hot.