Preheat the oven to 350 degrees F.
Add olive oil to a large skillet over medium heat.
Once hot, add the onions to the pan and lay them flat on their sides.
Cook on each side until browned, 3 to 4 minutes, being careful as you flip them.
Add the chicken stock, balsamic vinegar, rosemary, thyme and garlic to the skillet.
Add dots of butter around the skillet.
Turn the onions in the liquid so they are nice and coated.
Sprinkle with salt and pepper.
Place the skillet in the oven and roast for 25 minutes, stirring on occasion so the onions don't stick to the skillet.
Roast until the onions are nice and soft and the balsamic is caramelized.