Heat oven to 375° F. Spread the pecans on a rimmed baking sheet and toast in oven, tossing once, until fragrant, 6 to 8 minutes. Let cool, then roughly chop.
Fill a large saucepan with 1 inch of water and fit with a steamer basket. Steam the green beans until tender, 6 to 8 minutes; let cool. Whisk together the oil, honey, and mustard and season with ½ teaspoon each salt and pepper. Toss the beans with the dressing and sprinkle with the toasted pecans. Refrigerate for up to 8 hours. Just before serving, toss with the vinegar.