Heat a large stainless steel or cast iron pan over medium heat. Add olive oil to the pan & heat through, then add the butter & allow that to melt.
Working in batches, add onions & toss quickly to remove the moisture from the onions. Sauté, stirring occasionally, until the onions are soft & start to brown.
Sprinkle sugar & salt over the onions, stirring to combine. Reduce heat to medium-low & cook, stirring occasionally, until onions are light brown & begin to caramelize (~20 min).
Continue to cook, stirring frequently, until onions are uniformly brown & caramelized (~10 min). Remove pan from heat. Let the caramelized onions cool & use or store until ready to use.