On parchment-lined sheet pan, spread carrots in thin layer. Bake until tender-crisp, 25 to 35 minutes.
In stockpot over medium-high heat, bring vinegar to boil. Cook until reduced by one half. Add onion and salt. Return to a boil. Reduce heat. Simmer until onion is tender-crisp, 8 to 10 minutes.
Add roasted carrots to pan. Toss to coat and heat through.