Braising sweet onions -- here in a bit of chicken stock flavored with thyme and rosemary -- only intensifies their flavor.
Preheat oven to 350 degrees. Peel onions, trim root end, and cut each in half vertically; cut each half into three wedges. In a 10-inch cast-iron skillet, heat olive oil over medium heat. Add onion wedges, and season with salt and pepper; saute until golden brown, about 5 minutes on each side.
Add chicken stock, thyme, and rosemary; transfer pan to oven. Cook, basting onions periodically with the cooking liquid, until onions are tender and stock has reduced and thickened, 55 to 60 minutes. Remove from oven, and serve.