This is such an easy, yet elegant side dish that could compliment anything from a turkey dinner to a standing rib roast. I always set aside some beans, before they are tossed with the mushroom mixture, for the picky eaters in my family, but most kids do love the rich flavor of the mushrooms and shallots. Each part can be made ahead and refrigerated separately to help speed up meal preparation.
Heat a large pot of well-salted water to boiling. Meanwhile, trim green beans, cutting off the stem end. Prepare a large bowl of cold, icy water. When water is boiling, blanch beans for about 5 minutes or until crisp-tender. Immediately drain and plunge into the bowl of ice water, setting their deep green color and stopping any further cooking. Drain and set aside. Beans can be refrigerated at this point, wrapped in a paper towel and plastic bag, if desired.
Heat butter in a large skillet. Add shallots and sauté, stirring occasionally, until shallots are translucent, about 7 minutes. Add sliced mushrooms and salt and stir well to combine. Continue to cook, stirring occasionally, for 10 minutes more, or until the mushrooms have given off their liquid and it has evaporated. Set aside to cool, cover and refrigerate if desired.
To heat: Drizzle some olive oil in a large skillet over medium heat. Return mushrooms to warmed skillet. Add green beans and toss well. Sauté for about 8 minutes or until heated through. Season with salt and pepper and serve immediately.