French Onion Grilled Cheese

Heat the butter and olive oil in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until they are caramelized and golden brown, about 20-25 minutes. Season with salt and pepper to taste.
While the onions are cooking, preheat a separate skillet or griddle over medium heat.
Lay out 4 slices of bread on a clean surface. Spread a thin layer of Dijon mustard on each slice, if desired.
Divide the caramelized onions evenly among the 4 slices of bread, spreading them out in an even layer.
Sprinkle the shredded Gruyère cheese and mozzarella cheese over the caramelized onions.
Top each sandwich with the remaining 4 slices of bread to form sandwiches.
Place the sandwiches on the preheated skillet or griddle and cook for 3-4 minutes on each side, or until the bread is golden brown and the cheese is melted and gooey.
Once the sandwiches are cooked, remove them from the skillet and let them cool for a minute before slicing them in half.
Serve the French Onion Grilled Cheese sandwiches hot, and enjoy the delicious combination of flavors! 🌟