Preheat the oven to 400°
Pat thighs dry and season both sides liberally with salt and pepper, dried rosemary and onion powder. Set aside
In a saucepot, bring balsamic vinegar, orange juice, and brown sugar to a simmer. In a cup, mix cornstarch and water together and pour into the pan. Add ⅔ of the cranberries and continue to simmer until the sauce is sticky and the cranberries begin to burst. Add more water, 2 tbsp at a time, if the sauce gets too thick. Remove from heat and stir in mustard. Allow to cool slightly.
Line the bottom of a 13x9 baking dish with sliced onions, orange, rosemary sprigs, and garlic. Place chicken thighs on top, brush with cranberry glaze, and top with remaining cranberries
Bake for 25 minutes, glaze again, and cook an additional 10-15 minutes (or until the internal temperature has reached 165°), glazing every 5.
If desired, glaze one last time and broil for 1-2 minutes to caramelize and brown the tops of the thighs.
Allow to rest for 5 minutes and serve with rice, buttered noodles, orzo, roasted vegetables, or polenta.