Sandwich Ham dan Keju Panas dengan Bacon dan Bawang Karamel

Bahan-bahan (6 porsi)

• 6 slices bacon
• 1 yellow onion
• 8 tablespoons salted butter, divided
• 2 teaspoons Dijon mustard
• 1 teaspoon Worcestershire sauce
• 1 package King’s Hawaiian Hamburger Buns
• 2 pounds sliced deli ham
• 1 pound sliced Swiss (or any white melty) cheese