Combine the potatoes, garlic, red pepper flakes, 1 tablespoon vinegar and 1 tablespoon salt in a medium saucepan and cover with water. Bring to a boil over medium-high heat, then simmer, uncovered, until the potatoes are tender but not mushy, 7 to 10 minutes. Add the onion and cook until almost tender, about 1 more minute. Drain, discarding the garlic. Let the potatoes cool.

Whisk the remaining 4 tablespoons vinegar, the sugar and 1 teaspoon salt in a large bowl. Mix in the mayonnaise, sour cream and horseradish. Add the celery and pickles, then gently fold in the potatoes and chopped herbs. Season with salt and pepper. Cover and refrigerate until serving.

Photograph by Rick Lew