Simmer green beans in salted boiling water 5 minutes. Drain green beans and return skillet to moderate heat. Add oil and butter to the pan. Add onions and saute 2 to 3 minutes. Add mushrooms and season with salt and pepper. Saute mushrooms 3 to 5 minutes with onions, add green beans back to the pan. Heat green beans through and add sherry. Cook for 1 to 2 minutes. Transfer green beans and mushrooms to a serving plate.