Rinse beans then set aside
Melt butter in a deep non stick frying pan
Add onions and sauté until translucent
Sprinkle flour then cook for about 2 minutes stirring constantly
Add tomato sauce and salt to taste. Mix well then simmer over very low heat for about 5 to 7 minutes
Add beans, mix and cook for another 5 minutes, stirring occasionally
Serve hot