Melt 1 Tbsp. butter in large ovenproof skillet on medium-high heat. Add onions; cook 5 min., stirring frequently. Reduce heat to low; cook onions additional 25 min. or until golden brown, stirring occasionally and adding remaining butter as needed to prevent onions from drying out and sticking to bottom of skillet. Stir in garlic and 1 tsp. thyme; cook 5 min., stirring occasionally. Spoon into bowl.