Heat olive oil or butter in a large pan over medium-low heat.

Add the sliced red onions and stir to coat them in the oil.

Cook the onions slowly, stirring occasionally, for about 20-30 minutes until they become soft and golden brown.

Add the balsamic vinegar, sugar (if using), salt, and black pepper. Stir well and cook for another 5-10 minutes until the onions reach a deep caramelized color.

Remove from heat and let them cool slightly before using.

Serve on burgers, sandwiches, pizzas, or any dish that needs a boost of rich, sweet flavor!

Let me know if you want any modifications!