Heat the butter in a deep non-stick pan and sauté the onion till translucent. Add the chopped carrot, cabbage, potato and tomatoes and stir to mix. Add the flour and cook, stirring, on medium heat for one to two minutes.
Add the salt, pepper powder and four cups of water and bring the mixture to a boil. Lower the heat and simmer for five to seven minutes, or till all the vegetables are cooked. Serve piping hot.