Put green beans in a microwave-safe bowl with the water. Cover tightly and microwave on high for 4 minutes. Carefully remove the cover, drain in a colander, shaking off any excess water, and set aside.
While the beans are cooking, 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.