Heat oil in a large nonstick skillet over medium heat. Add onion, thyme, and bay leaf. Cover and cook 25 minutes, stirring occasionally.
Stir in salt and garlic; cook, uncovered, 2 minutes, stirring frequently. Stir in wine, brown sugar, and vinegar; bring to a boil. Reduce heat; simmer 12 minutes or until liquid almost evaporates and becomes syrupy.
Remove from heat; stir in pepper. Cool to room temperature. Discard bay leaf.