Start by washing the red potatoes thoroughly to remove any dirt. Cut them into quarters, ensuring they are roughly the same size for even cooking. Place them in a large pot and cover with water. Add a pinch of salt and bring the water to a boil. Reduce the heat and let simmer for about 10-15 minutes, or until the potatoes are tender when pierced with a fork.
While the potatoes are cooking, bring a smaller pot of salted water to a boil. Add the trimmed green beans and blanch them for about 3-5 minutes until they are bright green and tender-crisp. Immediately transfer them to a colander and rinse under cold water to stop the cooking process. This helps retain their vibrant color.
In a mixing bowl, combine the balsamic vinegar, Dijon mustard, fresh lemon juice, chopped garlic, Worcestershire sauce, and a pinch of salt and pepper. Whisk together until well blended. Gradually drizzle in the extra virgin olive oil while whisking continuously, creating a smooth and emulsified dressing.
Once the potatoes are cooked and drained, let them cool slightly. In a large mixing bowl, combine the warm potatoes, blanched green beans, and finely diced red onion. Drizzle the balsamic-dijon dressing over the salad and toss gently to coat all the ingredients evenly.
Taste and adjust the seasoning if necessary, adding more salt and pepper to your liking. You can serve the salad warm, at room temperature, or chill it in the refrigerator for about an hour to let the flavors meld.