Heat oven to 400º F. Spread the pecans on a rimmed baking sheet and bake, tossing once, until toasted, 6 to 8 minutes. Let cool, then roughly chop.
Meanwhile, bring a large pot of water to a boil and add 1 tablespoon salt. Add the green beans and cook until just tender, 4 to 5 minutes. Drain and run under cold water to cool.
In a large bowl, whisk together the oil, vinegar, mustard, maple syrup, ½ teaspoon salt, and ¼ teaspoon pepper. Add the green beans and pecans and toss to combine.