1. Fill a large saucepan with water and add 1 T. kosher salt. Bring to a boil and add the green beans. Cook for 3-4 minutes, or until they can easily be speared with a fork. Drain pasta water and plunge green beans into a bowl of ice water. This will stop them from cooking and help them stay crunchy.
2. In a large mixing bowl add vinegar, olive oil, mustard, sugar, salt, pepper and parsley. Whisk vigorously to make sure everything is thoroughly combined. Add drained green beans and toss well.
3. Cover bowl with plastic wrap and place in the fridge for at least 3 hours or up to overnight. Serve room temperature, or reheated in the microwave for 1 minutes or until desired temperature.