tablespoons sugar (if you like a sweeter dressing use 3 tablespoons)
teaspoon minced garlic
teaspoon salt, plus additional to taste
teaspoon Dijon mustard
Fresh ground black pepper, to taste
Set a large pot of water on the stove over high heat. Fill a large bowl about halfway with water and then fill with ice.
When water comes to a vigorous boil, toss in about 1 tablespoon salt and add the green beans. Boil for 2 to 3 minutes at most, pulling off the heat as soon as the green beans are crisp-tender. Quickly drain beans and transfer to the bowl of ice water. Allow green beans to soak in ice water bath while you prepare the dressing.
Combine dressing ingredients in a mason jar or other container with a tight-fitting lid. Cover tightly and shake for a good 30 seconds.
Transfer green beans to a colander and drain well. Cut into 1/2" pieces. Combine all the beans, celery, and onion in a large mixing bowl. Add dressing and toss to combine. Sprinkle salad with parsley and toss again. Transfer to an airtight container with a lid and refrigerate for at least 3 hours or overnight. Turn container a few times during marinating process to make sure all the ingredients get a good soak in the dressing.
When ready to serve, give it a taste and season with additional salt and pepper if you feel it needs it.