In large nonstick frying pan, melt butter and oil. Add onions and mushrooms and cook, covered, for about 3 to 7 minutes or until mushrooms start to soften. Remember, leave the lid on to keep it from drying out. When mushrooms have softened, add lemon juice along with seasonings and sour cream. Mix well. Add cooked pasta and heat thoroughly. Season with salt and pepper, if needed, and serve.