Place the green beans in a steamer insert placed in a pan above a small amount of boiling water. Cover and steam until tender but not squishy (or however you desire them to be).
While the beans are steaming, heat the oil over medium-high heat in a large nonstick skillet. Add the shallots and cook, stirring, until softened slightly, about 2 minutes. Add the mushrooms and cook, stirring occasionally, until the water they release has evaporated and they begin to brown, about 10 minutes. Add the green beans and stir to combine and rewarm. Season with salt and pepper and serve.