Wash and roughly chop the chadon beni, aka bandania, garlic, chive, and pimento pepper. Place them into a blender or food processor along with the black pepper, salt, mayonnaise, sour cream, sugar, lemon juice, and rice vinegar.
Blend until smooth and creamy, scraping down the sides as needed.
Taste and adjust the salt and sugar for balance—you’re aiming for a slightly sweet, creamy sauce with tangy, garlicky, and herbal notes.
Pour into a clean glass container, cover, and refrigerate for at least 1 hour before serving. Keep refrigerated and enjoy within 1 week. This sauce is best served chilled.