1. Boil the potatoes, carrots, and celery until soft. Drain, but reserve the water. Meanwhile, in a large saute pan, saute the onion in the margarine until soft and brown.
2. Add the cabbage to the onion and saute until the cabbage is slightly caramelized (it won't necessarily be soft yet).
3. Add 1/3 cup water to the cabbage/onion mixture, cover the pan, and let the cabbage steam until it's soft. When it's done, uncover the pan and saute until the water has evaporated.
4. Add the boiled vegetables and mash them lightly with a potato masher or the back of a heavy spoon.
5. Stir until the mixture is well-combined, adding water as needed to make it sort of pasty. Season with salt and pepper.
If you have leftovers, this is good for breakfast, too. Shape it into patties and cook a few minutes on both sides until golden brown.