Preheat oven to 375°F (190°C).
In a small bowl, whisk together balsamic vinegar, maple syrup, Dijon mustard, salt, pepper, and dried rosemary.
In a large oven-safe skillet, heat olive oil over medium heat. Add chicken thighs skin-side down and sear for 5-6 minutes until golden.
Flip chicken and add cranberries and sliced red onion to the pan.
Pour the balsamic mixture over the chicken and transfer the skillet to the oven.
Roast for 25-30 minutes, or until chicken is fully cooked (165°F internal temp).
Optional: To thicken sauce, remove chicken and stir in a slurry made with arrowroot and water over low heat until thickened.
Spoon sauce over chicken, garnish with fresh rosemary, and serve hot.