Scrub the potatoes to remove dirt, then cut into 1-inch chunks and place in a large pot. Cover with cold water and add kosher salt. Bring to a gentle boil over medium-high heat.

Reduce heat and simmer for about 10-15 minutes, or until just tender. Drain and let steam dry.

In a medium bowl, whisk together olive oil, lemon juice, vinegar, sumac, mustard, salt, and pepper until smooth.

Transfer warm potatoes to a mixing bowl. Drizzle half dressing and toss until coated. Let sit for 5-10 minutes.

Fold in onion, parsley, dill, and mint (if using) and drizzle remaining dressing. Gently fold to combine.

Cover and chill in the refrigerator for at least 20-30 minutes before serving. Stir gently and sprinkle with more sumac before serving.