Melt butter in large saucepan over medium heat. Add onion,cook and stir until tender. Add flour,salt, pepper, cook stirring constantly for one minute. Add milk. Increase heat to medium-high and bring to boil, stirring constantly. Boil and stir 1 minute. Add corn and sausage, return to boil. Reduce heat to medium-low and simmer 10 minutes. Stir in potatoes and heat through. Top each serving with parsley and cheese, if desired.