French Onion Soup Grilled Cheese Sandwich

In a large skillet, heat olive oil and butter over medium heat. Add sliced onions and cook, stirring occasionally, until softened and translucent, about 10-15 minutes.

Reduce heat to low and continue cooking, stirring frequently, until the onions are deeply caramelized and golden brown, about 15-20 minutes. Be patient, this step is key to flavor!

If the pan gets too dry, deglaze it with the beef broth (optional). Add the brown sugar, balsamic vinegar, thyme, salt, and pepper. Stir well to combine and cook for another minute.

Spread half of the bread slices with a generous layer of shredded Gruyere cheese. Top with a portion of the caramelized onions, then add another layer of Gruyere cheese. Finish by topping with the remaining bread slices.

Melt butter in a griddle or large pan over medium-low heat. Place sandwiches in the pan and cook for 3-4 minutes per side, or until golden brown and the cheese is melted and gooey.

Remove from the pan, slice in half (optional), and serve immediately.