In a small bowl, mix together balsamic vinegar, 2 tablespoons olive oil, red wine vinegar, honey, salt, and pepper.
Add chicken thighs to the marinade, then toss to combine and refrigerate for 30 minutes.
Coat a rimmed baking sheet with remaining 2 tablespoons olive oil. Add red onions, then scatter chicken thighs between them, pouring the extra marinade on top of the onions. Pour on the cranberries. Place a few sprigs of fresh tarragon on top,
Roast for 20 to 25 minutes, or until juice from chicken run clear (when thickest part reads at least 165 F on a meat thermometer). Serve immediately.