1. Fill a large skillet with 1/2 inch water; bring to a boil, and salt generously. Prepare a large bowl of ice water.

2. Add green beans to skillet; cover, and steam until crisp-tender, 5 to 10 minutes (time will depend on size and freshness of green beans). Transfer to ice water to cool; drain and pat dry (if making ahead, cover and refrigerate up to 1 day). Wipe skillet dry.

3. In same skillet, heat butter over medium-high. Add mushrooms and shallots; season with salt and pepper. Cover, and cook, stirring occasionally, until mushrooms release their liquid, 3 to 5 minutes. Uncover, and continue to cook, tossing occasionally, until browned, 8 to 12 minutes more.

4. Add green beans; cook just to heat through, 1 to 2 minutes.