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