Place potatoes in a large pot filled with cold water to cover; season with salt. Bring to a boil; reduce heat and simmer until tender, 15–18 minutes. Drain and place potatoes back in the pot. Cover with clean dish towel for 15 minutes.
Meanwhile, heat ¼ cup oil in a medium skillet over medium-high. Add onion; season with salt. Cook, stirring occasionally, until soft, about 5 minutes.
Transfer onion to a large bowl. Stir in vinegar, mustard, Old Bay, black pepper, and cayenne. Add potatoes, parsley, and chives to vinaigrette and toss to combine. Taste and season with salt and black pepper if needed; drizzle with more oil. Enjoy!
