Place the quartered potatoes in a large pot and cover them with cold water. Add 1 tablespoon of salt and bring to a boil over high heat. Reduce heat and simmer until the potatoes are fork-tender, about 15-20 minutes. Drain immediately and let them cool slightly (about 5 minutes).

While the potatoes cook, prepare the vinaigrette. In a small bowl or jar, whisk together the olive oil, lemon juice, dried oregano, minced garlic, Dijon mustard, 1 teaspoon of salt, and pepper until emulsified.

While the potatoes are still warm (this helps them absorb the dressing), place them in a large mixing bowl. Pour about half of the vinaigrette over the warm potatoes and gently toss to coat evenly. Allow the potatoes to sit for 10 minutes to soak up the flavor.

Add the thinly sliced red onion, Kalamata olives, and diced celery to the bowl with the potatoes. Pour the remaining vinaigrette over the mixture and toss gently until everything is combined.

Fold in the crumbled feta cheese and fresh dill. Taste the salad and adjust seasoning, adding more salt or lemon juice if necessary.

For best flavor, cover and chill the salad in the refrigerator for at least 30 minutes before serving, allowing the flavors to meld completely. Serve chilled or at room temperature.