Bring 1 inch of water to a boil in a large pot fitted with a steamer basket. Fill a large bowl with ice water; set aside.
Place green beans, stem ends trimmed, in basket. Cover; steam until crisp-tender, 5 to 8 minutes. Plunge beans in ice water. When cool, drain; pat dry with paper towels. Transfer to a large bowl.
In a small bowl or jar, whisk or shake extra-virgin olive oil, white-wine vinegar, Dijon mustard, coarse salt, and ground pepper until thickened and combined. Pour over beans; toss to coat.