1. Heat butter in a 12-inch nonstick skillet over medium heat until hot. Add onions, cook 8 to 10 minutes or until tender and beginning to brown, stirring occasionally. Stir in wine, plums, preserves and vinegar, cook about 5 minutes or until liquid is evaporated and marmalade thickens and turns dark, stirring occasionally. Remove from heat; stir in thyme and salt. Cool slightly and serve warm.