Preheat oven to 400°F. Slice the top off the garlic head, drizzle with olive oil, wrap in foil, and roast for 35 to 40 minutes until soft. Cool slightly.
Meanwhile, boil potatoes in salted water until fork-tender, about 12 to 15 minutes. Drain and let cool.
Squeeze roasted garlic cloves into a bowl and mash into a paste.
Whisk together roasted garlic, mayonnaise, sour cream, Dijon mustard, vinegar, salt, and pepper.
Add potatoes, celery, green onions, and parsley. Toss gently until evenly coated. Chill before serving for the best flavor.
Notes
Plain Greek yogurt can be used instead of sour cream.
Yukon Gold potatoes can replace the red potatoes.
Add crumbled bacon, chopped hard-boiled eggs, fresh dill, or chives for a different flavor.
Prepare the potato salad up to one day ahead and keep it refrigerated until serving.
Store leftovers in an airtight container in the refrigerator for up to 3 days.
Freezing is not recommended because the potatoes can become grainy and the dressing may separate.