Preparation Make-ahead tip: Sprouts can be trimmed one day in advance. Cook the sprouts and onions just before serving. Preheat oven to 350°F. Warm oil in large, heavy, ovenproof skillet over medium heat. Add sprouts; cook 5 minutes, stirring, until slightly golden. Add onions; cook 2 minutes, stirring, until golden. Pour broth into skillet to reach halfway up side of vegetables. Stir in thyme, salt, and pepper. Bring broth to a simmer; transfer pan to oven. Roast 15 to 20 minutes, until a sharp knife easily slides through a sprout. Transfer to serving bowl; serve hot.