In a large pot of salted boiling water, cook green beans until crisp-tender, about 3 minutes. Transfer beans to a colander and rinse under cold water to stop cooking. Set aside. In a large bowl, whisk together oil, mustard, garlic, and 1 teaspoon salt. Add onions, herbs, lemon zest, and reserved green beans and toss to coat. Refrigerate for at least 2 hours or up to overnight. To serve, remove and discard garlic, add lemon juice, and toss well.