1. Saute onions in hot butter over medium heat (in a saucepan) for 5 minutes. Sprinkle with flour and cook for 1 more minute.
2. Add milk, potatoes, salt and pepper, stir well. Cover and cook for 20-25 minutes, stirring occasionally (until potatoes are tender but not mushy).
3. Add cheese and sausage and cook and stir until cheese is melted.