While you're setting up to cook, slice the red onions very thinly into half-moons and set aside. Chop the green onions, parsley, and dill, keeping them in separate piles for easy access. Mince the garlic cloves fresh (I find freshly minced garlic makes a noticeably sharper, more vibrant dressing than pre-minced). Pit and halve the olives, and drain the capers. Zest the lemon and measure out all your dry ingredients and oils so everything is ready when you need it.
Cut the potatoes into 1-inch cubes for even cooking—this size ensures they'll cook through in about 15-20 minutes without falling apart. Bring a large pot of water to a boil, add 1 tablespoon of salt, then add the potato cubes. Maintain a gentle boil until the potatoes are fork-tender but still hold their shape. Drain in a colander and let them cool for about 5 minutes while you prepare the dressing—they should still be warm when you dress them.
In a bowl, whisk together the red wine vinegar, Dijon mustard, dried oregano, minced garlic, 1 teaspoon salt, and freshly ground black pepper until well combined. Slowly drizzle in the extra virgin olive oil while whisking to emulsify the dressing. I like to taste and adjust the seasoning here—a good vinaigrette should be punchy and balanced. The warm potatoes will absorb these flavors better than cool ones, which is why we're making this while they finish cooking.
Transfer the warm cooked potatoes from Step 2 to a large bowl. Add the sliced red onions, green onions, parsley, dill, and olives with capers. Pour the vinaigrette from Step 3 over everything and toss gently but thoroughly to coat all the potatoes evenly. Scatter the lemon zest over the top and fold it in gently. The warm potatoes will absorb the dressing beautifully, creating a flavorful, well-seasoned salad.
Let the salad sit for 10-15 minutes to allow the flavors to meld together and the potatoes to fully absorb the dressing. You can serve it warm right away, or let it cool to room temperature—both are delicious. Taste one more time and adjust seasoning if needed before serving.