Melt the margarine in a medium-sized soup pan. Add onions, carrots, and peppers and cook over moderate heat until the onions are soft.
Add in the potatoes, seasonings, corn, and water to the mixture already in the pot. Bring contents to a full boil. Reduce the heat and simmer for 20 minutes. Add the Brussels sprouts at that time. Cover and simmer for another 10 minutes.
In a separate bowl, stir milk, flour, and cold water together until smooth. Add to the soup and stir in. Keep the soup simmering, stirring for another 3 minutes.