In a large skillet heat olive oil over medium heat.
Add onions, season with salt and stir to coat with olive oil.
Cook onions over medium heat, stirring occasionally to avoid burning until onions are brown and caramelized, about 20 minutes.
Stir in balsamic vinegar and cook for about 2 minutes.
Remove from heat and set aside.
Prepare spread
In a medium bowl stir mayonnaise, worcestershire sauce, garlic powder, parsley flakes and thyme until combined. Store in the refrigerator until ready to serve.
Prepare grilled cheese
Heat a large skillet over medium heat.
Butter outside pieces of bread.
Inside the sandwich, spread both slices with mayonnaise spread. Top with 2 slices of cheese and caramelized onions.
Cook sandwich about 4-5 minutes per side until bread is toasted and golden brown and cheese is melted.