Look for baby turnips with the greens on them for an elegant look that will wow. Trim greens to about 1/2 inch from the top of the bulb; then slice turnips vertically.
Combine first 9 ingredients in a medium saucepan; bring to a boil. Reduce heat; simmer 5 minutes. Remove from heat; cool 10 minutes. Place turnip and onion in a bowl; pour vinegar mixture over turnip mixture. Cool completely. Serve immediately, or cover tightly and refrigerate up to 1 week.