Bring a large pot of salted water to a boil. While you are waiting on the water, cut the potatoes into 2 inch pieces and cut the fresh green beans into 2 inch lengths. Add the potatoes to the water and cook until just tender, 10-15 minutes. Use a strainer to remove them to drain. Add the green beans to the water and cook until just tender, about 5 minutes and then remove with a strainer to drain. Rinse the beans and the potatoes with cold water to stop the cooking and set aside.
Meanwhile, in a large bowl, whisk together the mustard, vinegar, sugar, oil and dried herbs until you have formed an emulsion. Stir in the salt and pepper and red pepper flakes and then taste the dressing to adjust the acidity. If it’s too vinegary add more oil. Stir in the onion, bell pepper, and olives then add the potatoes and green beans, along with the parsley to the bowl. Toss together with clean hands. Taste to adjust seasoning. This can be served immediately or it will hold in the fridge or up to 3 days.