Cook noodles according to package. Melt butter in a large skillet, add onions and saute on high for 3 minutes, add mushrooms and saute for additional 3 minutes. Reduce heat to medium, add kielbasa and cook for 10 minutes. Reduce heat to low, add green pepper. When noodles are done, reserve one cup of water, drain remaining liquid; add noodles and reserved water to skillet. Heat through, salt and pepper to taste.