Peel and slice onions into thin slices. You can adjust thickness based on preference, but the thinner, the faster they will cook.
Heat a pan over medium-high heat. Add the oil, and allow it to come to temperature.
Add onions to pan, and sautee, stirring on occasion until they begin to brown and get color, approx. 8-10 minutes.
To fully caramelize the onions naturally, continue cooking until thick and jammy, maybe 10-15 minutes more. To save some time, you can (optional) add 1 tsp of sugar and cook for another 2-4 minutes.
Add the remaining French onion soup mix to the caramelized onions and saute another 2 minutes. Once done, remove from the heat and set aside.