In a skillet, heat 2 tablespoons of olive oil over medium heat. Add 3 thinly sliced onions along with a pinch of salt, and cook for 5–7 minutes until they become translucent. Stir occasionally to ensure even cooking.

To the translucent onions, sprinkle 1 tablespoon of brown sugar and drizzle 1 tablespoon of balsamic vinegar. Stir well and reduce the heat to low. Continue cooking for an additional 15–20 minutes.

In a separate skillet, place 6 slices of bacon and cook over medium heat for 6–8 minutes until crispy. Transfer to a paper towel-lined plate to drain excess grease.

Preheat a griddle or a nonstick skillet over medium-low heat. Gather your ingredients and butter, ensuring the bread is ready for assembly.

Spread softened butter generously on one side of each of the 4 slices of bread. Lay the buttered sides down, and layer cheese, caramelized onions, and crispy bacon.

Place the assembled sandwiches onto the preheated griddle. Cook for 3–4 minutes until the bottom slice is golden-brown and cheese begins to melt. Flip and grill for another 3–4 minutes.

Remove the sandwiches from the griddle and allow them to rest for 1 minute before slicing. Serve warm.