Cook the beetroot: Boil whole beetroot in water until tender (about 30–45 minutes). Let cool, then peel and slice or grate.
Prepare the dressing: In a small pot, heat vinegar, sugar, salt, pepper, and optional spices until sugar dissolves.
Combine: Place beetroot and chopped onion in a bowl, then pour over the warm dressing.
Mix and chill: Stir well and refrigerate for at least 1–2 hours (overnight is best for flavour).
Servir: Serve cold as a side dish.