Heat olive oil in a large skillet over medium heat. Add sliced onions and cook, stirring occasionally, until softened, about 10 minutes.
Reduce heat to low and continue cooking, stirring occasionally, until the onions are deeply caramelized and golden brown, about 30-40 minutes. Be patient, this process takes time to develop the sweetness.
Stir in balsamic vinegar and brown sugar. Cook for another 5 minutes, stirring constantly, until the vinegar has reduced and the onions are glazed.
Season with salt and pepper to taste. Serve warm or at room temperature.
Notes
These caramelized onions are delicious served on burgers, sandwiches, pizzas, or as a side dish with grilled meats.