In a medium skillet, crumble sausage and fry over medium-high heat until browned. Drain grease. Preheat oven to 350 degrees F (175 degrees C).
In an 8x8 inch casserole dish, mix together sausage, mushroom soup, and green beans. Mix in 1 cup sour cream if desired. Cover dish with foil and bake for 30 minutes. If you like, sprinkle 1 cup shredded cheese over top and return to oven, uncovered, for 5 minutes or until cheese melts.