Place potatoes in a large pot filled with cold water to cover; add 2 tablespoons salt. Bring to a boil; reduce heat and simmer until tender, 15–18 minutes. Drain and place potatoes back in the pot and cover with a clean dish towel for 15 minutes.
Meanwhile, heat ¼ cup oil in a medium skillet over medium-high. Add onions and cook, stirring occasionally, until soft, about 5 minutes. Transfer onions to a large bowl.
In a separate bowl, stir in the warm olive oil used to cook the onions in, vinegar, mustard, 1 teaspoon of salt, black pepper, and cayenne; mix to combine. Drizzle vinaigrette over warm potatoes. Add parsley, and chives and toss to combine. Taste and season with salt and black pepper, if needed. Finish with a little olive oil. Serve immediately. Enjoy!
