Heat oven to 400 degrees. On a large rimmed baking sheet, toss the potatoes, 2 tablespoons olive oil and a pinch each of salt and pepper. Arrange in an even layer and cook until golden-brown and crisp, tossing about halfway through, about 35 minutes.
Let the potatoes cool for 5 minutes, then add them to a large bowl with the lemon juice. Toss to coat, then let cool to room temperature.
Add the tomatoes, olives, dill, parsley, pepperoncini and remaining 1 tablespoon olive oil to the bowl. Stir gently to combine, then sprinkle the feta over the top. Taste and add more lemon, salt and pepper if desired. Serve room temperature or cold.