1Melt butter in 10-inch skillet over medium-low heat. Cook onions in butter 15 to 20 minutes, stirring occasionally, until onions are brown and caramelized; remove from heat.
2Heat oven to 350ºF. Place half of the cheese on bottom half of focaccia. Top with onions and remaining cheese. Replace top of focaccia. Wrap tightly in aluminum foil. Bake 15 to 20 minutes or until cheese is melted. Cool 5 minutes. Cut into wedges.