Mix ground beef with Worcestershire sauce in bowl.
Add garlic powder, thyme, salt, and pepper.
Form mixture into four equal patties.
Heat cast iron skillet over medium-high heat.
Cook onions in butter until golden brown.
Remove onions and set aside.
Cook burger patties for 4 minutes per side.
Add cheese slices during last minute.
Toast buns lightly in same skillet.
Spread mayo and mustard on bun bottoms.
Layer patty, cheese, and caramelized onions.
Top with remaining bun half.