Whisk together the olive oil, Dijon mustard, sherry vinegar, minced shallot, minced garlic, and salt in a bowl until fully emulsified. This dressing should be made ahead so the flavors can meld together. While the vinaigrette sits, prep all your vegetables: trim and cut the green beans into 1-inch pieces, thinly slice the red onion into half-moons, chop the fresh parsley and chives, and measure out the fresh thyme leaves. Having everything ready before cooking ensures the hot potatoes will be dressed while still warm, which allows them to absorb the vinaigrette's flavors more effectively.

Bring a large pot of water to a boil, then add the salt and fingerling potatoes cut into 1/2-inch thick coins. Boil for about 8-10 minutes until the potatoes are just tender when pierced with a fork—they should be cooked through but still hold their shape. In the final 1-2 minutes of cooking, add the green beans to the same pot so they cook briefly and stay bright green and crisp-tender. Drain everything immediately in a colander and return to the warm pot.

While the potatoes and green beans are still warm, pour the vinaigrette from Step 1 over them and gently toss to combine. I find that dressing hot vegetables allows them to absorb the flavors much more deeply than if you wait for them to cool. Let the mixture sit for a few minutes so the potatoes can soak up the vinaigrette, then transfer to a serving bowl.

Once the salad has cooled to room temperature, fold in the fresh parsley, thyme, chives, and sliced red onion from Step 1. Gently mix everything together so the herbs and onions are evenly distributed without breaking the potatoes. Taste the salad and season generously with freshly ground black pepper. I like to add the herbs at this point rather than earlier because their fresh flavor stays more vibrant when not exposed to the warm vinaigrette for too long.

Refrigerate the salad for at least 30 minutes (or up to 4 hours) before serving to allow all the flavors to meld together and the salad to chill completely. Give it a gentle stir before plating, adding any additional pepper to taste if desired.