Combine onion, vinegar, sugar, cinnamon stick, peppercorns and cloves in a large saucepan.
Bring to a boil over medium-high heat.
Cover, reduce heat to medium-low and simmer until the onion is tender-crisp, 4 to 6 minutes.
Stir in beets.
Transfer to a large bowl and let marinate, stirring occasionally, for 30 minutes.
NOTES:
Make Ahead Tip: Cover and refrigerate for up to 1 week.
How to Prep & Steam Beets: Trim greens (if any) and root end; peel the skin with a vegetable peeler. Cut beets into 1/2- to 1-inch-thick cubes, wedges or slices.
To steam on the stovetop: Place in a steamer basket over 1 inch of boiling water in a large pot. Cover and steam over high heat until tender, 10 to 15 minutes.
To steam in the microwave: Place in a glass baking dish, add 2 tablespoons water, cover tightly and microwave on High until tender, 8 to 10 minutes. Let stand, covered, for 5 minutes.