In large saucepan combine beans, water, onions, and tomatoes. Cook over medium heat for 20 minutes. Put through sieve or puree in blender. Set mixture aside. In same saucepan, melt butter; add flour to bubbling. Add a small amount of the pureed mixture to saucepan; heat and stir. Add remaining bean mixture. Salt and pepper to taste. Simmer 5 minutes before serving. Serves 6.