Drain beets and reserve 2/3 cup liquid.
Tie pickling spices in a piece of cheesecloth.
Combine beets, sliced cucumber, onion, reserved liquid, sugar, vinegar and cheesecloth bag in a deep, 2-quart, heat-resistant, non-metallic casserole.
Heat, covered, on full power 12 minutes or until mixture boils.
Chill to marinate.