Grilled Onions:
In medium low skillet, melt butter.
Add onions and cook, stirring regularly until onions are tender and golden, about 30 minutes.
Stir in worcestershire sauce and cook for another 10-15 minutes.
Sauce:
Combine all sauce ingredients in small bowl. Whisk together until thoroughly combined.
Sandwich:
Heat cast iron grill pan over medium heat.
Butter one side of each slice of bread.
Place bread butter side down on hot grill.
Place cheese on bread and grill until melted.
Top with grilled onions and sauce.
Top with second piece of grilled cheesy bread and continue to grill over low heat until cheese is fully melted.
Enjoy!