In 12-inch nonstick skillet, melt butter over medium-high heat. Stir in onions to coat with butter. Cook uncovered 10 minutes, stirring every 3 to 4 minutes.
Reduce heat to medium-low. Sprinkle salt over onions. Cook 35 to 40 minutes longer, stirring well every 5 minutes, until onions are deep golden brown (onions will shrink during cooking).