Preparation / Directions:
Bring vinegar to a boil in a cast iron skillet over medium-high heat. Remove from heat; place onions, cut side down, in skillet.
Bake at 400 degrees for 55 to 60 minutes or until onions are tender and vinegar turns the color of dark chocolate.
Melt butter in a skillet over medium heat. Add walnuts; cook 2 minutes, stirring often. Add sugar and next 3 ingredients; cook until bubbly, stirring often.
Place on onion half, cut side up, on each plate. Sprinkle walnut mixture evenly around onions; drizzle onions with vinegar. Serve immediately.