Trim the green beans and cut them into shorter pieces if desired.
Bring a large pot of salted water to a boil.
Add the green beans and cook for 3 to 5 minutes, or until bright green and crisp-tender.
Drain the green beans and pat them dry with a clean kitchen towel.
In a small jar or bowl, add the lemon juice, extra virgin olive oil, Dijon mustard, minced garlic, salt, and pepper.
Shake or whisk until the dressing is combined and slightly thickened.
Arrange the green beans on a serving plate or add them to a bowl.
Top with the diced tomato and finely diced red onion.
Pour the dressing over the salad.
Garnish with finely chopped parsley and serve.