Heat a medium non-stick pan over medium-low heat and melt butter.
Add onions and cook until caramelized, stirring occasionally, approximately 20 minutes. Transfer to a plate and tent to keep warm.
In a large bowl, combine ground beef, Worcestershire sauce, salt, and pepper. Form 4 square patties, slightly larger than the bread and approximately ¼ inch thick.
Wipe the pan clean with a paper towel and place over medium-high heat. Cook the patties in batches, about 1-2 minutes per side. Remove patties from the skillet and set aside.