The mushrooms give this dish an unctuousness that's hard to resist.
Bring a large saucepan of water to a boil; add salt. Prepare an ice bath; set aside. Working in two batches, cook the green beans in boiling water until crisp-tender, 3 to 5 minutes. Using a slotted spoon, plunge beans into ice bath to stop the cooking. Drain, and pat dry; set aside.
Heat 1/2 cup oil in a large skillet over high heat until hot but not smoking. Working in batches so as not to crowd skillet, add mushrooms, and season with salt and pepper. Cook, stirring frequently, until golden and crisp, 2 to 3 minutes. Using a slotted spoon, transfer mushrooms to paper towels to drain. Add more oil as needed for subsequent batches.
Reduce heat to medium. Add beans and remaining 1 tablespoon oil; season with salt and pepper. Cook, tossing, until the beans are heated through, about 3 minutes. Transfer to a large serving dish. Add mushrooms; toss to combine. Serve warm or at room temperature.